# Role of Large-Scale Traveling Ionospheric Disturbances in the Positive Storm Phase Observed by the Millstone Hill Radar and GNSS TEC Measurements

G P Geethakumari, A T Aikio, L Cai, H Vanhamäki, I I Virtanen, A Coster, A Maute, Aurélie Marchaudon, P.-L -l Bleilly

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## ► To cite this version:

G P Geethakumari, A T Aikio, L Cai, H Vanhamäki, I I Virtanen, et al., Role of Large-Scale Traveling Ionospheric Disturbances in the Positive Storm Phase Observed by the Millstone Hill Radar and GNSS TEC Measurements. Journal of Geophysical Research Space Physics, 2026, 131 (1), pp.e2025JA034562. 10.1029/2025ja034562. hal-05441676

HAL Id: hal-05441676

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Submitted on 5 Jan 2026

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# JGR Space Physics

## RESEARCH ARTICLE

10.1029/2025JA034562

**Key Points:**

* Increased Joule heating at high latitudes during a geomagnetic storm launched two large-scale traveling ionospheric disturbances (LSTIDs)

* These equatorward propagating LSTIDs played a dominant role in causing the positive storm phase at mid-latitudes

* LSTIDs caused the mid-latitude F-region to move up and down and produced increases in total electron content

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

G. P. Geethakumari,
@gopika.prasannakumara@oulu.fi

**Citation:**

Geethakumari, G. P., Aikio, A. T., Cai, L., Vanhamäki, H., Virtanen, I. I., Coster, A., et al. (2026). Role of large-scale traveling ionospheric disturbances in the positive storm phase observed by the Millstone Hill radar and GNSS TEC measurements.

*Journal of Geophysical Research: Space Physics, 131*, e2025JA034562. https://doi.org/10.1029/2025JA034562

Received 14 AUG 2025

Accepted 17 DEC 2025

© 2026. The Author(s).
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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## Role of Large-Scale Traveling Ionospheric Disturbances in the Positive Storm Phase Observed by the Millstone Hill Radar and GNSS TEC Measurements

G. P. Geethakumari<fnref n="1" /> ID, A. T. Aikio<fnref n="1" /> ID, L. Cai<fnref n="1" /> ID, H. Vanhamäki<fnref n="1" /> ID, I. I. Virtanen<fnref n="1" /> ID, A. Coster² ID, A. Maute<fnref n="3" /> ID, A. Marchaudon<fnref n="4" /> ID, and P.-L. Bleelly<fnref n="4" /> ID

**Abstract** A moderate geomagnetic storm was driven by high-speed solar wind stream on 14 March 2016. We show that large-scale traveling ionospheric disturbances (LSTIDs) played a significant role in producing the ionospheric storm positive phase at mid-latitudes in the North American sector. The equatorward expansion of the positive storm phase followed the equatorward propagation of the LSTIDs, after which the total electron content (TEC) increased by 11 TECU (42%). Our novel method to estimate Joule heating suggests that sudden increases in Joule heating in the auroral oval triggered the LSTIDs. The effects of the LSTIDs observed by the Millstone Hill radar were sudden uplifts of the ionospheric F region followed by downlifts. The absence of an eastward electric field in the radar measurements rules out the role of electric field in causing the positive storm phase. We suggest that the uplifts of the ionosphere were associated with equatorward neutral wind perturbations carried along with the LSTIDs, whereas the downlifts were associated with poleward winds. During the whole period of the two LSTIDs, the TEC continued to increase. The increase in TEC during the uplift can be explained by the decrease in the recombination rate at higher altitudes under continuous solar photoionization. The maximum in peak-F density during the second downlift can be explained by compression of the plasma. To explain the increase in TEC during the downlift, an additional mechanism is needed, which could be downward plasma flux from the plasmasphere or increase in atomic oxygen due to changes in thermospheric circulation.

**Plain Language Summary** Ionospheric storms are solar wind driven variations in the ionized region of the Earth's upper atmosphere. Ionospheric storms can have negative and positive phases, which correspond to decreases and increases in the peak electron density and in the column integrated electron density called total electron content (TEC). In specific, the positive storm phase is poorly understood. In this paper, we show that the large-scale traveling ionospheric disturbances (LSTIDs) launched by Joule heating at high latitudes play a significant role in producing the ionospheric storm positive phase at mid-latitudes on 14 March 2016. The Global Navigation Satellite System TEC measurements in the North American sector showed a southward expansion of the positive storm phase followed by equatorward propagation of the LSTIDs with TEC increasing by 42%. During the overpasses of the LSTIDs over Millstone Hill, the radar measured abrupt uplifts and downlifts of the ionosphere and electron density enhancements. Under photoionized conditions, the uplift caused by the perturbed southward wind increases plasma production under reduced recombination. During downlift produced by poleward wind, electron density enhancement is caused by plasma compression, but TEC enhancement requires additional mechanisms like plasmaspheric influx or downwelling of oxygen atoms.

## 1. Introduction

The Earth's ionosphere is a highly dynamic system that is primarily controlled by solar radiation and the continuously streaming solar wind. Variations in the parameters of the solar wind that interact with the Earth's magnetosphere can greatly affect the inter-coupled magnetosphere-ionosphere-thermosphere system. When the solar wind interacts with the magnetosphere, significant amounts of energy and momentum are transferred to the magnetosphere, initiating geomagnetic storms (Gonzalez et al., 1994; Perreault & Akasofu, 1978). These storms, in turn, induce large-scale changes in the ionosphere and thermosphere, with effects that can persist from hours to several days. A well-known impact of geomagnetic storms on the ionosphere is the variation in electron density, referred to as an ionospheric storm (Borries et al., 2015; Buonsanto, 1999; Prölss, 1997, 2008).

<footnote n="1">¹Space Physics and Astronomy, University of Oulu, Oulu, Finland, ²MIT Haystack Observatory, Millstone Hill, MA, USA,</footnote>

<footnote n="3">³Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA,</footnote>

<footnote n="4">⁴IRAP, Université de Toulouse, CNRS, CNES, Toulouse, France</footnote>
Depending on several factors such as the phase of the geomagnetic storm, latitude, magnetic local time (MLT) and season, the ionospheric storm can either cause an increase or decrease in the F-region electron density from its quiet time level (W. Liu et al., 2017), called positive or negative ionospheric storm, respectively (Buonsanto, 1999; Tsagouri et al., 2000; Prölss, 2008, and references therein). Often, during the beginning of the geomagnetic storm's main phase, mid-latitudes show an initial positive ionospheric storm, followed by a long-duration negative storm (Mendillo, 2006).

Of the two different phases of the ionospheric storm, the mechanisms leading to the negative phase are better understood compared to the positive phase (Buonsanto, 1999; Mendillo, 2006; Prölss, 1997). Various studies have investigated negative storms using the total electron content (TEC), F-region electron density ($N_e$), and the O/N$_2$ ratio. A decrease in the O/N$_2$ ratio, owing to the neutral composition changes induced by Joule heating at high latitudes, is directly related to the depletion of the electron density due to the increased dissociative recombination rate (Geethakumari et al., 2024; Prölss, 1976; Y. Zhang et al., 2004, and references therein). It has been suggested that increase in Joule heating at high latitudes would affect the global circulation, causing equatorward winds, which can transport the neutral composition disturbance to mid-latitudes (Prölss, 2008). In addition to the changes in the neutral composition, also increases in ion temperature by enhanced convection electric field at high latitudes and sub-auroral polarization streams (SAPS) may play a key role in causing the negative ionospheric storm phase (Ellahouny et al., 2024; Geethakumari et al., 2024; Grandin et al., 2019).

The mechanisms driving positive ionospheric storm phases are not well understood (Burns et al., 2007; Prölss, 2008). Several different mechanisms may act together and the primary mechanism may also vary between different events. There are two mechanisms that can uplift the plasma to higher altitudes, where the recombination rate is slower, leading to enhanced electron densities and enhanced TEC. One of them is an eastward electric field (Foster & Rich, 1998; Huang, 2008; Huang et al., 2005; Lei et al., 2008; Swisdak et al., 2006) and the other is an equatorward wind (Prölss, 1997; Werner et al., 1999). Huang et al. (2005) reported a positive ionospheric storm in the morning sector at the Millstone Hill station that lasted for more than 10 hr. Using the Millstone Hill radar data along with the Global Navigation Satellite System (GNSS) TEC, they suggested that the driving mechanism for the positive storm phase was primarily an eastward electric field with contribution from southward neutral winds. A simulation study combined with observations from GNSS TEC and Millstone Hill radar on the ionospheric response to another geomagnetic storm event was carried out by Lei et al. (2008). Their study showed that the positive ionospheric storm phase observed during the initial phase of the geomagnetic storm was caused by eastward electric field. Most of the papers discussing the role of a southward wind are modeling studies (e.g., Balan et al., 2009; Lin et al., 2005; Lu et al., 2008), since the observational evidence for this mechanism is sparse. Kim et al. (2023) reported observational evidence of thermospheric winds contributing to positive ionospheric storm phase at mid-latitudes.

The role of traveling atmospheric disturbances (TADs) and the connected feature of traveling ionospheric disturbances (TIDs) in driving positive ionospheric storm phase has also been reported by several authors (e.g., Afraimovich et al., 2000; Bauske & Prölss, 1997; Goncharenko et al., 2007; Habarulema et al., 2013; Lu et al., 2001). According to Prölss (1997), short-duration positive ionospheric storm phases are primarily driven by TADs, generated by increased Joule heating at high latitudes (Nykiel et al., 2024; Richmond, 1979). As the TADs propagate to lower latitudes, they carry along with them moderate (~150 m/s) equatorward winds (Bauske & Prölss, 1997), which may push the plasma to higher altitudes along magnetic field lines where recombination rate is low. Intense and sudden dissipation of energy at high latitudes in the form of Joule heating may generate atmospheric gravity waves (AGW, Richmond, 1978) and at some distance from the source region, these waves superpose to evolve into impulse like disturbances called TADs (Bauske & Prölss, 1997). TADs are coupled through ion-neutral interaction to TIDs.

Other mechanisms that have been suggested to cause a positive ionospheric storm phase are plasmaspheric influx of charged particles (Burns et al., 2008; Y. Cai et al., 2022; Li et al., 2018; Park, 1974) and increase in ionization due to increase in atomic oxygen owing to the downwelling of atomic oxygen to F-region at mid and low latitudes. Pedatella et al. (2009) concluded in an event study that refilling of the topside ionosphere and plasmasphere was not the primary mechanism for producing the long-lasting positive storm phase during the studied event. Aa et al. (2021) reported that the 12-hr positive storm phase could collectively be generated by atomic oxygen downwelling caused by storm-time thermospheric circulation, combined with plasma uplifting due to the equatorward neutral wind and enhanced storm-time TAD/TIDs.
A study conducted by Geethakumari et al. (2024) characterized the ionospheric response to a prolonged moderate geomagnetic storm, driven by a high speed solar wind stream (HSS) and associated stream interaction region (SIR), near the equinox on 14–20 March 2016 for magnetic latitudes (MLAT) 40°–90° in the northern hemisphere. Their study focused on the negative ionospheric storm phase at high and mid-latitudes that persisted for several days. The storm caused 25%–50% depletion in TEC, accompanied by a concurrent decrease in the height-integrated O/N₂ ratio, highlighting the role of neutral composition changes in the negative ionospheric storm. Positive phase storm effects in TEC at mid-latitudes were also shown in the paper, but not discussed in detail.

The goal of this paper is to study the strong positive phase observed in the American sector at the beginning of the storm reported by Geethakumari et al. (2024). In this study, we use the Millstone Hill incoherent scatter radar measurements together with the GNSS TEC observations. The focus is on the effect of the observed large-scale traveling ionospheric disturbances (LSTIDs) on the development of the positive phase. The observational evidence presented in this study clearly demonstrates the dominant role of LSTIDs in driving the positive ionospheric storm. We also apply a new method to estimate the Joule heating at high latitudes, which is a key driver for mid-latitude changes and can excite LSTIDs. The paper is organized as follows. Section 2 explains the data and associated methodology to analyze the different data sets used in the study. Section 3 shows the results of the Millstone Hill radar, TEC, and LSTID analysis during the storm. Joule heating as the source of LSTIDs and the effects caused by LSTIDs on the ionosphere are discussed in Section 4. Section 5 gives conclusions.

## 2. Data and Data Analysis

### 2.1. GNSS Data for TEC and LSTID Analysis

The GNSS TEC data, available in the Madrigal database, are used to study the changes in TEC during the HSS/SIR driven geomagnetic storm that commenced on 14 March 2016. The TEC maps are given in the geographical coordinate system with a spatial resolution of 1° by 1° in every 5 min (Rideout & Coster, 2006; Vierinen et al., 2016). For discerning the storm induced changes in TEC, we followed the same method as in Geethakumari et al. (2024) in which the background reference TEC is removed from the original measured TEC. The background reference TEC in Geethakumari et al. (2024) is constructed by taking the average of the four quietest days of the month, which were March 5, 13, 25 and 26. However, in this study, 5 March is excluded from the quiet day average as the TEC on this day exhibited perturbations at mid-latitudes. Hence, the background reference TEC is estimated by taking the average of TEC on March 13, 25 and 26. The TEC values obtained after removing the quiet-day background TEC are hereafter called DTEC in this paper.

To study the LSTID waves, the line-of-sight (LOS) GNSS TEC data provided by the Madrigal database are used to derive the detrended TEC maps using the method by Zhang et al. (2019). The detrended TEC (dTEC) is calculated by subtracting the background TEC from the LOS TEC. Estimation of the background TEC is done by applying the low-pass Savitzky-Golay filter (Savitzky & Golay, 1964) to the LOS TEC with a linear basis function, a time window of 60 min and the polynomial order 1. The LOS TEC data with an elevation angle less than 30° are removed to avoid the effect of ground-satellite ray paths. The derived detrended TECs are binned into 0.1° × .25° geographic maps to identify LSTIDs.

### 2.2. Millstone Hill Incoherent Scatter Radar Data and Analysis

The Millstone Hill incoherent scatter radar provides the measurements of electron density, electron and ion temperatures, and the line-of-sight ion velocity. The radar facility is located at geographic latitude 42.6°N and longitude 71.5°W (geomagnetic latitude MLAT= 52.03°). The Millstone Hill radar system consists of a fixed vertically directed antenna (zenith) and one fully steerable antenna called the Millstone Hill Steerable Antenna (MISA, Foster & Erickson, 2000). On 13–15 March 2016, the MISA antenna scanned from northwest to northeast with the azimuth angles of −40.5° (MISA1), −13.5° (MISA2) and 13.5° (MISA3), and the elevation angle was 45° for all beams. The sequence of one measurement cycle is zenith–MISA1–zenith–MISA2–zenith–MISA3, and the total duration is 22 min 22 s. The 960 µs long pulse mode is used for the first zenith beam, and both alternating code and 480 µs single pulse are used for the remaining beams. Our study focuses on the F-region responses. Hence, only the 480 µs single pulse measurements are analyzed, which provide sufficient signal-to-noise ratio, altitude coverage, and altitude resolution for our study. The integration time for the measurement is 3 min in the zenith direction and 4 min for each of the MISA positions. As a result, the consecutive measurement points of the zenith beams with 480 µs single pulses are separated by 7 and 15 min alternatively. Finally, line-of-sight
velocities between 200 and 400 km altitudes from the zenith and MISA beams during each scanning cycle are used to estimate the full ion velocity vector and electric field, by means of statistical inversion (Nygrén et al., 2011). The time resolution is the same as the cycle duration.

## 2.3. Solar Wind Data and Other Measurements

In this study, solar wind parameters such as speed, interplanetary magnetic field (IMF) components, and geomagnetic indices including the SYM-H and AE indices were obtained from the OMNIWeb database (King & Papitashvili, 2005). The data utilized in the analysis have a time resolution of 5 min and the solar wind parameters have been delayed to the Earth's bow shock. Using these measurements, we estimated the Newell coupling function

$$NCF = v^{4/3} B_T^{2/3} \sin^{8/3} \left(\frac{\theta}{2}\right), \qquad (1)$$

where v is the solar wind velocity, $B_T$ is the transverse magnetic field and $\theta$ is the IMF clock angle in the Geocentric Solar Magnetospheric (GSM) coordinate system (Newell et al., 2006).

The Defense Meteorological Satellite Program (DMSP) provides us with the cross-track ion velocities from the Special Sensor for Ions, Electrons, and Scintillation (Rich & Hairston, 1994) and auroral images are obtained from the Special Sensor Ultraviolet Spectrographic Imagers (SSUSI, Paxton et al., 2002).

## 2.4. Joule Heating Estimate

To estimate the Joule heating source for ionospheric disturbances, we implement a novel data-based method. A brief description on the estimation method is given in Appendix A and a more extensive paper about the method is under preparation. Compared to some previous methods (e.g., Billett et al., 2018), our method does not rely on statistical conductivity or wind models, but we use measured magnetic fields by SuperMAG and AMPERE, in addition to plasma convection from SuperDARN. This allows us to study dynamical, localized variations in Joule heating.

Ignoring the neutral wind, the height integrated Joule heating is calculated as

$$Q_J = \vec{E} \cdot \vec{J} \qquad (2)$$

Here $\vec{E}$ is the convection electric field and $\vec{J}$ is the height-integrated horizontal current density. The convection electric field is derived from the SuperDARN potential maps (Greenwald et al., 1995; Ruohoniemi & Baker, 1998; Shepherd & Ruohoniemi, 2000). The height-integrated current density is calculated as a sum of the curl-free (CF) and divergence-free (DF) parts. The DF part of the current is obtained from the gridded ground magnetic disturbance provided by SuperMAG (Waters et al., 2015). We use the gridded AMPERE magnetic field data product (Anderson et al., 2002; Waters et al., 2001, 2020) to estimate the CF part of the ionospheric current.

# 3. Observations and Results

## 3.1. Solar Wind Driver

The temporal variations in the solar wind, associated geomagnetic field conditions and ionospheric TEC responses from 16:00 UT on March 14 to 11:00 UT on 15 March 2016 are shown in Figure 1. Figure 1a shows the IMF, with the total magnetic field ($B_t$) depicted by the black line, the GSM southward component ($B_z$) in red and the eastward component ($B_y$) in blue. Around 17:10 UT, an increase in $B_t$ is observed, with a peak of 20 nT, together with abrupt fluctuations in the polarity of $B_z$. This compression in the IMF and the fluctuations are typical signatures of an SIR (Jian et al., 2006). At 19:00 UT, $B_z$ reaches a minimum of -18 nT. However, $B_y$ changes its polarity to negative initially at 18:50 UT and remains negative for about 2 hr and later fluctuates. The IMF $B_y$ stays mostly negative for a longer period of time after 19:00 UT. Meanwhile, $B_z$ fluctuates until 00 UT on 15 March.
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Solar wind, geomagnetic and ionospheric variations during 14 March 2016, 16 UT–15 March 2016, 11 UT, (a) IMF B<sub>z</sub> (red), B<sub>y</sub> (blue), B<sub>t</sub> (black), (b) Solar wind speed, (c) Newell coupling function (NCF), (d) Auroral Electrojet (AE) index, (e) SYM-H index and (f) DTEC at mid-latitudes (40°–60° MLAT) averaged over 09–21 MLT sector.</figcaption></figure>

Figure 1b shows the solar wind speed. A gradual increase in the solar wind speed from the background level of 380 km/s is observed starting around 17:20 UT, and reaching a peak of 624 km/s by 23:05 UT on March 14. The wind speed stays above 500 km/s during the studied interval, recovering to the background level on 16 March, which is not shown in the figure. Due to its prolonged high values and IMF characteristics typically shown by HSS/SIR, this solar wind stream can be identified as a high speed solar wind stream (Denton & Borovsky, 2012; Grandin et al., 2019; Kavanagh et al., 2012).

Figure 1c shows the Newell coupling function (NCF) during the storm. The NCF does not show any variations during the initial rise in the HSS/SIR until 19 UT on 14 March, despite the increase in the solar wind velocity as well as the magnetic field. A sudden increase in NCF is observed at 19 UT simultaneously with the southward turning of IMF B<sub>z</sub>, indicating that solar wind–magnetosphere coupling is greatly enhanced due to increased reconnection at the dayside magnetopause. The NCF stays generally high during the studied interval, with short-duration decreases (typically less than 20 min) due to northward turnings of the IMF B<sub>z</sub>.

The auroral electrojet (AE) index (Davis & Sugiura, 1966) increases with the southward turning of the IMF B<sub>z</sub> at 19 UT, as seen in Figure 1d, similar to the NCF. It reaches a maximum of about 900 nT several times during the storm. These elevated AE index values are related to the high intensity long duration continuous auroral activity (HILDCAAs, Tsurutani and Gonzalez (1987)) and increased electrojet currents in the auroral oval, that lasted for 7 days during the geomagnetic storm as reported in Geethakumari et al. (2024).

Figure 1e shows the SYM-H index, which indicates the strength of geomagnetic storms (Wanliss & Showalter, 2006). With the arrival of the HSS/SIR at 17:20 UT, SYM-H shows a sudden rise from its quiet time level, caused by the compression of the magnetopause due to increased solar wind dynamic pressure. This marks the start of the initial phase of a geomagnetic storm, known as sudden storm commencement (SSC, Curto et al., 2007). The increase in SYM-H index persisted for nearly 4 hours, peaking at 50 nT at 18:40 UT. Subsequently, the SYM-H index declined to quiet level and remained stable for a short period. By 21:15 UT, it continued to decrease, marking the onset of the main phase, and reached a minimum of –60 nT at 07:20 UT on 15 March, indicating a moderate geomagnetic storm (Gonzalez et al., 1994). Overall, the storm had multiple SYM-H minima and lasted for 7 days (see Geethakumari et al., 2024).
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Keograms showing the positive ionospheric storm phase at the longitude sector 65°W–80°W spanning 40°–85° MLAT between 17 UT on 14 March and 03 UT on 15 March 2016. (a) Observed TEC, (b) Reference TEC (mean TEC of 3 quietest days of March 2016) and (c) DTEC obtained after removing reference TEC from observed TEC. The horizontal black dashed lines indicate the Millstone Hill location at 52.03° MLAT.</figcaption></figure>

Figure 1f shows an overview of the temporal evolution of DTEC (Observed TEC–Reference TEC, explained in Section 2.1), averaged over 40°–60° MLAT and 09–21 MLT (the area is taken into account in the average). Compared to the reference level TEC, an increase of 2 TECU in TEC is observed during the SSC, as shown by the DTEC curve. Following that, as the SYM-H index decreases, DTEC begins to rise further and intensifies with the main phase onset at 21:15 UT, reaching a maximum of 8 TECU at 23:30 UT. Later, as the main phase is still progressing, the enhancement in DTEC is seen to subside, reaching a local minimum of depletion at 03:10 UT. After this dip in the DTEC, a slow enhancement in DTEC is observed, this time reaching a maximum of 4 TECU after the first SYM-H minima at 07:20 UT. In this paper, we focus on the enhancement in TEC seen between 20 and 03 UT, which is the first positive phase of the storm.

## 3.2. GNSS TEC

Figure 2 shows keograms of the observed TEC (panel a), reference TEC (mean of the three quietest days of the month as explained in Section 2.1, panel b), and their difference DTEC (panel c), from 40°–85° MLAT latitudes before and during the positive phase of the storm. Data are averaged over the geographic longitude sector 65°–80°W covering the eastern parts of North America, centered around Millstone Hill longitude (71.5°W). The black horizontal dashed line shows the latitude of the Millstone Hill radar.

The yellow to red colors in Figure 2c indicate enhancements in TEC whereas blue shades indicate TEC depletions, compared to the reference TEC. After the arrival of the SIR at 17:20 UT, some enhancement in DTEC is seen at polar and mid-latitudes between 17:00 and 20:00 UT. After 20:00 UT, this enhancement further intensifies. A maximum of 10 TECU is observed between 22:25 UT and 23:00 UT around 60° MLAT, as indicated by the dark red color in Figure 2c. In addition to the TEC enhancement, an expansion of positive storm effects to lower latitudes is also observed.

It is also noted that a sharp boundary of TEC enhancement is seen in Figures 2a and 2c near 70° MLAT that later shifts to 60° MLAT. A full video of polar DTEC maps is shown in the supplementary material of Geethakumari et al. (2024).

Figure 3 shows a selected polar plot of DTEC (left panel) and the auroral emission in Lyman-Birge-Hopfield Short (LBHS) from DMSP/SSUSI measurements (right panel). Overlaid on both panels are the DMSP cross track ion velocity. The white star indicates the location of Millstone Hill radar. In Figure 3a, the DMSP cross track velocity shows anti-sunward flows over the polar cap and sunward flows within the auroral and sub-auroral regions. Figure 3b shows that near 15 MLT, the strongest sunward flows are located equatorward of the LBHS auroral emissions, in the sub-auroral region. Hence, we can identify that the sharp DTEC boundary
<figure><img src="image_6.png" /><figcaption>**Figure 3.** An instance showing SAPS and associated TEC depletion, both presented in AACGM MLT-MLAT coordinate system. Panel (a) shows the cross track ion velocity measured by DMSP F16 satellite overlaid on DTEC maps and panel (b) shows the same cross track ion velocity overlaid on SSUSI LBHS emission measurements. The white star indicates the location of Millstone Hill incoherent scatter radar.</figcaption></figure>

describes the equatorward edge of the sub-auroral region and the strong flows in the sub-auroral region can be identified as sub-auroral polarization streams (SAPS, Foster & Burke, 2002).

### 3.3. Millstone Hill Incoherent Scatter Radar Observations

For a comprehensive study of the positive ionospheric storm phase at mid-latitudes, we choose the Millstone Hill incoherent scatter radar facility located at 42.6°N 71.5°W, 52.03° MLAT on the east coast of the North American sector, which is centered to 65°–80°W sector, for which the TEC is shown in Figure 2. Figure 4 shows measurements of plasma parameters from Millstone Hill radar in the vertical/zenith direction for 13 March (panels a to c) and 14 March (panels d to f). In the zenith direction, the successive measurement points are separated by 7 and 15 min (Section 2.2). Unlike the GNSS TEC maps, the Millstone Hill radar had no measurements on the chosen quiet days of the month, except for 13 March from 20 UT onward. Therefore, the period from 20 UT on 13 March to 03 UT on 14 March, referred as the quiet day, is selected as the reference time for analyzing the radar data. The parameters shown in the figure are electron density ($N_e$) (Figures 4a and 4d), electron temperature ($T_e$) (Figures 4b and 4e) and line-of-sight ion velocity ($V_i$), positive upwards (Figures 4c and 4f).

Figures 4a and 4d show the $N_e$ variations on the quiet and storm day respectively. Overlaid on both panels are the respective $N_e$ contours. On the quiet day, a gradual decay of $N_e$ is observed with increasing solar zenith angle in the post-Noon sector, which is part of the diurnal variation. However, on the storm day, an enhancement in $N_e$ can be clearly observed after 20 UT, compared to the same time on the quiet day. It is also noted that there is a gradual uplift of the F-layer after 20 UT, which will be discussed in detail later in this section.

In addition to the gradual rise of the ionosphere to higher altitudes, sudden and transient uplifts of F-layer are also observed during the ongoing positive phase of the storm. The first small uplift is observed around 19:40 UT and it will be discussed more in Section 3.4. After that, two significant uplifts are observed. The first of those is observed at 20:43 UT and is marked with a black vertical dashed line in Figure 4 and denoted T1. This uplift is followed by a gradual increase in $N_e$, which is seen between 250 and 350 km and reaches a peak density around 21:30 UT. The second and more distinct uplift occurs at 21:58 UT (T2, marked with a black vertical dashed line) and continues till 22:15 UT, with an ascend of 55 km. Again, the uplift is followed by an enhancement in $N_e$ after 22:15 UT. Subsequently, a slow descent of the F-layer is seen between 22:20–23:10 UT together with a strong enhancement in $N_e$ which reaches a maximum around 23:00 UT. The increase in $N_e$ persists even after sunset, which occurs at 00:20 UT at 300 km altitude (marked by the red dotted vertical line), and continues until approximately 03:00 UT the following day.

Figures 4b and 4e show the $T_e$ variations on the quiet and storm day, respectively. Between 20:00 and 01:00 UT on the quiet day, following the local noon, a gradual decrease in $T_e$ at all altitudes with time is observed, which is typical during sunset hours (Zhang & Holt, 2004). On the storm day, a decrease in $T_e$ is seen at altitudes above
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Plasma parameters measured by Millstone Hill incoherent scatter radar in the zenith direction. On the top are the plasma parameters between 20 UT on 13 March and 03 UT on 14 March and on the bottom are the measurements from 16 UT on 14 March to 03 UT on 15 March. (a) & (d) Electron density, (b) & (e) Electron temperature, (c) & (f) Ion velocity (positive upwards). The white line on (d) represent the TEC calculated from the Millstone Hill radar electron density measurements. The vertical black dashed lines indicate the sudden uplifts of the F-layer at 20:43 UT (T1) and 21:58 UT (T2). The vertical red dashed line indicates the sunset time at 300 km altitude.</figcaption></figure>

300 km, compared to quiet time values at the same local time, see Figure 4e. The electron temperature is reduced by 500–1000 K above 300 km during the positive storm phase. During this time, $N_e$ also increased from the background condition, as discussed above. Therefore, the decrease in $T_e$ could be associated with an increase in electron density under a constant energy input (Wang et al., 2006). In addition, an increase in electron density means more collisions, leading to an increase in cooling rate and thus a decrease in $T_e$ (Wang et al., 2006; Zhang & Holt, 2004).
The vertical line-of-sight ion velocity measurements are shown in Figures 4c and 4f representing quiet day and storm day, respectively. The sign convention is such that a positive velocity means the ions are moving upwards. As shown in Figure 4f, there is an overall enhancement in the line-of-sight ion velocities on storm day compared to the quiet day, more significantly at altitudes above 425 km, until 22:20 UT. At 20:43 UT, when the uplift T1 is observed in the $N_e$, a simultaneous increase in upward ion velocity is also noted. Similarly, at 21:58 UT, a transient but stronger upward ion velocity is observed simultaneously with the prominent F-layer uplift (T2). After 22:20 UT, the ion velocity is reduced or even reversed in direction at times. It is during this time that the slow descend of the ionosphere and the associated peaking of electron density are observed.

As discussed previously, a gradual uplift of the ionosphere is seen along with the two sudden uplifts after 20:00 UT during the positive storm phase compared to the quiet day. This is further analyzed by comparing the individual electron density profiles on the quiet and storm days. Figure 5 shows selected electron density profiles on the quiet day (dashed curves) and the storm day (solid curves) around the same UT times.

In Figure 5a, the profiles on the storm day show an enhancement in electron density at altitudes above 230 km compared to the quiet day profiles together with an upward shift in the peak height. At altitudes below 220 km, a slight decrease in electron density is observed in the profile at 20:28 UT (green curve). Moreover, a small increase in $N_e$ peak is observed in the profile at 20:28 UT (green curve) compared to the profile at 20:21 UT (blue curve). This indicates that there is gradual uplift in the ionosphere. In Figure 5b, the $N_e$ profiles on the storm day (solid lines) are uplifted compared to the quiet day (dashed lines), which can easily be seen at altitudes below 240 km. The peak altitudes are significantly higher.

In addition to the rise of the ionosphere compared to the quiet time, two sudden transient uplifts are observed during the ongoing positive phase, as discussed above. During the uplift T1 in Figure 5b, the peak altitude rises (black curve) and electron densities at the peak altitude and above the peak are increased compared to the situation before the uplift (red curve). The effect of the uplift T2 on electron density profiles is shown in Figure 5c. The $N_e$ profile at 22:13 UT on the storm day (green solid curve) shows a significant increase of about 55 km in peak height as well as a decrease in electron density at altitudes below the peak compared to the profile at 21:50 UT (blue solid curve). Figure 5d shows two profiles during the descent following the uplift T2. The profile at 22:42 UT (red curve) has peak altitude of 300 km, which is smaller than at 22:13 UT (340 km, green curve in panel c), and the peak electron density has increased. The black curve in panel d, 15 min after the red curve, shows that the descending continues with peak altitude decreasing to about 285 km. At the same time, the peak electron density continues to increase, reaching a peak value of $9.5 \times 10^{11} m^{-3}$.

### 3.4. Large Scale Traveling Ionospheric Disturbances

To analyze traveling ionospheric disturbances during the positive storm phase, GNSS TEC data is detrended using the methodology explained in Section 2.1. A video of dTEC maps is provided in the supplementary material Movie S1 and Figure 6 shows selected snapshots from the video. The data coverage is sparse at latitudes above 50° and below 20°N. The yellow-red shades of the color scale indicate the wave crests and blue colors indicate the wave troughs. The white star indicates the location of the Millstone Hill radar.

Figure 6 shows the propagation of two distinct TIDs that are sequentially launched. Figures 6a and 6b show the propagation of the first TID. In Figure 6a, we can see a large crest of the TID in the American sector between 40°–50°N which propagates almost due south as seen in Figure 6b. The amplitude of the disturbance is between 0.2 and 0.4 TECU and has a wavelength of about 2,000 km (20°). TIDs with horizontal wavelengths larger than 1,000 km fall in the category of large scale TIDs (LSTIDs) (Afraimovich et al., 2000; Hocke & Schlegel, 1996; Hunsucker, 1982; Kersley & Hughes, 1989; Nykiel et al., 2024; Shiokawa et al., 2002). As seen in the supplementary video Movie S1, this TID overpassed Millstone Hill around 20:43 UT, see also discussions below.

A second TID that follows immediately behind the first one can be seen in Figures 6c–6f. The trough of the wave is observed between 40°–50°N, see panels c and d. It arrived near the radar location around 21:58 UT, see also the supplementary video, Movie S1. Trailing behind the trough is a crest with a higher amplitude >0.4 TECU, see Figures 6d–6f. The second TID has an oblique propagation direction unlike the first TID which propagated due south. Propagating in the southwest direction with an azimuth of 20° from the south, the second TID has a wavelength of >1,000 km, typical for LSTIDs.
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Comparison of the electron density profiles at Millstone Hill during the storm time on 14 March (solid lines) and during the quiet time on 13 March (dashed lines) in four selected time intervals, illustrating storm-time ionospheric uplift: (a) 20:20-20:30 UT, (b) 20:40-21:15 UT, (c) 21:45-22:15 UT, and (d) 22:40 -23:00 UT. The times for the curves refer to the middle of the 3-min integration period.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 6.** A 2-D distribution of detrended TEC map over American sector showing the propagation of two distinct LSTIDs during the main phase of the geomagnetic storm. (a–b) Propagation of the first LSTID. (c–f) Propagation of the second LSTID which is propagating in the south-west direction. The white star indicates the location of Millstone Hill radar.</figcaption></figure>

To understand the role of LSTIDs in the positive phase of the ionospheric storm, a one-to-one comparison of dTEC (detrended TEC) with DTEC (observed TEC–reference TEC) at different longitudes at the mid-latitudes (40°–60° MLAT) in the American sector is conducted, as shown in Figure 7. The figure starts with Millstone Hill longitude 288° and sweeps west of Millstone Hill until 265°. The first column, Figures 7a–7e, depicts the keogram of dTEC at geographic longitudes 288°, 280°, 275°, 270° and 265° and the second column, Figures 7f–7j correspondingly shows the keogram of DTEC at these longitudes. The horizontal dashed lines in Figures 7a and 7f indicate the magnetic latitude of Millstone Hill.

In Figures 7a–7e, around 20 UT, a weak simultaneous (vertically aligned) enhancement in dTEC is observed at all latitudes and all longitudes. S.-R. Zhang et al. (2023) reported a similar kind of global oscillation during disturbed times and named it simultaneous global ionospheric density disturbance that are likely caused by penetration electric fields. No significant changes in DTEC are observed at this time. However, the small uplift in the ionosphere observed between 19:40 UT and 20:00 UT in Figure 4d is simultaneous to this global oscillation.

After 20:30 UT, dTEC in Figures 7a–7e shows oblique decreases and enhancements that correspond to propagating troughs (blue) and crests (red) in the LSTID waves. The second LSTID has a larger amplitude than the first one, and it starts with a wave trough. The slope of the trough is marked with a solid oblique line and the slope of
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Panels (a–e) show the dTEC keogram with signatures of TIDs at different longitudes to the west of Millstone Hill. Panels (f–j) show the DTEC keograms at the same longitudes showing the expansion of the positive storm phase from high to mid-latitudes. The black oblique lines show the slopes of the LSTIDs, solid lines marked at the leading boundaries of the wave troughs and dashed lines marked at leading boundaries of wave crests. The dashed vertical line shows the arrival times of the LSTIDs at Millstone Hill.</figcaption></figure>

the following crest with a dashed oblique line, both marked at the leading boundaries of the troughs and crests. For the first LSTID, only the crest (dashed line) is easy to identify. However, also the first LSTID starts with a trough and produces an uplift of the F-region, similar to the second LSTID. The start time of the uplift at Millstone Hill helps us to find the trough in the keogram for the first LSTID and to mark it with a solid line. The slope for the trough is taken similar to the crest, since they both propagate with the velocity of the first LSTID.

The first LSTID reaches the Millstone Hill location at 20:43 UT (T1, marked with a dashed vertical line). Since the first LSTID front is almost east-west aligned and propagates southward, it appears at all the longitudes in the American sector around the same time. From the slope of this propagating disturbance, the velocity of this LSTID is 490 m/s. The reported velocities of LSTIDs range from 400 to 1,000 m/s (e.g., Afraimovich et al., 2000; Borries et al., 2009; Shiokawa et al., 2002). Comparing to the DTEC keograms, Figures 7f–7j, we can notice a similar expansion of the TEC enhancement to lower latitudes with time. The similar slopes of dTEC and DTEC, observed at all the longitudes shown in Figure 7, suggest that the propagation of the LSTID contributes to the equatorward expansion of the positive ionospheric storm.

The wave trough of the second TEC perturbation arrives at the Millstone Hill location at 21:58 UT (T2, marked with a dashed vertical line), as seen in Figure 7a. The arrival of the LSTID is simultaneous to the abrupt uplift of the F-layer observed in the Millstone Hill electron density measurements. It propagated in the southwest direction with an angle of 20° from the south. The estimated meridional velocity of the LSTID is 610 m/s and phase velocity is 575 m/s (estimated following the method suggested in Chen et al. (2020)). A similar slope is observed in the DTEC keograms at all longitudes. During the passage of the second LSTID, DTEC at all longitudes show peak enhancement.

As we can see from Figure 4d, the uplifts of the F-layer are simultaneous with the arrivals of the troughs of the LSTIDs ($T1 = 20:43$ UT and $T2 = 21:58$ UT). The arrivals of the crests for the two LSTIDs occur at 21:15 UT and 22:20 UT, respectively and they approximately coincide with the starts of the downlifts. The uplift and the following enhancement in the F-region electron density are greatest during the arrival of the second LSTID with the greatest amplitude. The maximum enhancement in F-region electron density is observed between 22:30 and 23:00 UT during the second downlift. During the whole period of the LSTIDs the TEC continued to increase.
<figure><img src="image_11.png" /><figcaption>**Figure 8.** (a) Polar map of Joule heating at 19:50 UT and (b) at 21:20 UT. (c) keogram of Joule heating averaged over 13–22 MLT. (d) keogram of detrended TEC showing LSTID over 275° geographic longitude. The red dashed vertical lines (a) and (b) in panel (c) indicate the corresponding time of Joule heating as shown in panels (a) and (b). The oblique solid lines show the slopes of the LSTIDs and the horizontal dashed line indicates the Millstone Hill location at 52.03° MLAT.</figcaption></figure>

# 4. Discussion

## 4.1. Joule Heating as a Source of LSTIDs

Previous studies have suggested that LSTIDs often originate in regions of intense Joule heating (Hunsucker, 1982; Richmond, 1978; Sheng et al., 2020; Shiokawa et al., 2007). Any major step-like change in energy deposition in the thermosphere can drive atmospheric gravity waves (Borries et al., 2009; Förster & Jakowski, 2000) and TADs. Further, the neutral wind perturbations associated with TADs can be transferred to the plasma medium via ion-neutral collisions, and be manifested in the ionosphere as TIDs (Hunsucker, 1982; Lei et al., 2008; B. Liu et al., 2025). Shiokawa et al. (2007) reported TID events at mid-latitudes in the Japanese sector during a major geomagnetic storm. Using assimilative mapping of ionospheric electrodynamics (AMIE) analysis and TIE-GCM simulation, they reported that Joule heating enhancements at high latitudes triggered TADs, which in turn generated the observed TIDs. Using a combination of GNSS TEC and EISCAT incoherent scatter radar measurements, Nykiel et al. (2024) reported a case of multiple LSTIDs in the European sector both in the night time and daytime during disturbed conditions. They concluded that the night time LSTIDs are primarily driven by high latitude Joule heating, whereas both Joule heating as well as particle precipitation in the cusp could trigger daytime LSTIDs.

Figure 8 shows the Joule heating source regions that launched the two LSTIDs. Panels a and b show the source regions in the polar plots and panel c shows the keogram of the Joule heating. Using the method explained in Section 2.4 and in Appendix A, we estimated the Joule heating during this storm. To find if high latitude Joule heating drives the observed LSTIDs, backtracing of the LSTID wavefronts is performed on the keogram of Joule heating as shown in Figures 8c and 8d. In Figure 8d, the detrended TEC (dTEC) over the geographic longitude 275° is shown for the latitude range of 40°–60° MLAT (from Figure 7c). The dTEC keogram over Millstone Hill (288°) was not chosen due to the sparsity of data. Since the propagation direction of the first LSTID was due south, the arrival time of the wavefront was simultaneous at all longitudes. Whereas, for the second LSTID, due to its oblique direction in propagation there was a time lag of approximately 1 min between the arrival times at
Millstone Hill longitude and at 275° longitude. Thus, the selection of the longitude for backtracing the LSTID with the Joule heating is justifiable. For the purpose of backtracing the LSTIDs, keogram of Joule heating, averaged over 13–22 MLT sector and extending from 60 to 80° MLAT, Figure 8c, is combined with Figure 8d. The black horizontal dashed line indicates Millstone Hill magnetic latitude (52.03° MLAT).

In Figure 8d, the slopes of the LSTID wavefronts, the black solid lines, are extended to latitudes above 60° MLAT in Figure 8c. The backtraced slope of the first LSTID is found to coincide with the abrupt intensification in Joule heating at 19:50 UT, originating from above 65° MLAT as seen in Figure 8c. Figure 8a shows the polar plot of Joule heating at 19:50 UT. The white star on the polar plot indicates the location of the Millstone Hill radar. An increase in Joule heating is observed above 65° MLAT at 19:50 UT. Between 13 and 16 MLT, Joule heating is above 12 mW/m² and during this time Millstone Hill is in the post noon sector (15 MLT). The propagation direction of the LSTID oriented due south further supports that the Joule heating in the post noon sector triggered the LSTID observed around 20:43 UT near Millstone Hill.

Similarly, the backtraced slope of the second LSTID coincides with the sudden intensification in Joule heating at 21:20 UT around 65° MLAT, see Figure 8c. The source region of the LSTID is identified using the polar plot of Joule heating (Figure 8b), which shows the Joule heating at 21:20 UT. Following the main phase onset, Joule heating increases in all MLT sectors, most notably in 16–21 MLT sector. In 18–21 MLT sector, Joule heating exceeds 20 mW/m², indicating a likely source region for the LSTID generation. This region of intense Joule heating is northeast to the radar, agreeing with the propagation of the LSTID in the southwest direction.

## 4.2. LSTID Induced Variations in the Electron Density Profiles and TEC

The positive storm phase covers the whole North American continent with a sharp poleward boundary corresponding to the sub-auroral region showing SAPS (Figure 3) and further expands to lower latitudes following the propagation of the LSTIDs. As seen in Figures 7f–7j, this positive storm phase starts gradually and by 20:43 UT (T1) the enhancement in TEC has reached 4 TECU at longitude 280°. Also, in the Millstone Hill radar measurements (Figure 4d), a gradual uplift of the ionosphere together with an enhancement in $N_e$ after 20 UT is noticed, compared to the quiet time conditions (Figure 4a). We speculate that this initial enhancement in TEC and $N_e$ and the gradual uplift of the ionosphere is caused by the enhancement of the equatorward wind after the storm onset. However, after the arrival of the first LSTID, the enhancement in TEC reaches above 6 TECU and it further rises and reaches a maximum of 11 TECU (42% enhancement) during the overpass of the second LSTID. Thus it is very evident that the large increases in TEC and $N_e$ are associated with the passage of the LSTIDs. This shows that the equatorward propagating LSTIDs play a dominant role in the positive ionospheric storm.

As discussed in Section 3.4, the sudden transient uplifts of the ionosphere and associated $N_e$ enhancements observed in the Millstone Hill radar measurements on 14 March 2016 are simultaneous with the arrival of the two LSTIDs at 20:43 UT and 21:58 UT. The $N_e$ profiles shown in Figure 5b (black curve) and Figure 5c (green curve) clearly show the uplift of the F-layer and the $N_e$ enhancement. The uplift at 21:58 UT is more pronounced, with an ascend of about 55 km, when the second LSTID with higher amplitude passed over Millstone Hill, see Figure 5c. The vertical line-of-sight ion velocity also enhanced during the observed time of uplifts. Stronger upward ion drifts, reaching above 50 m/s, are observed with the arrival of the second LSTID, shown in Figure 4f.

Figure 9 shows the northward (panel a) and eastward (panel b) electric field components and field-aligned ion velocities (panel c, $V_\|$ positive upwards) derived from Millstone Hill radar measurements as explained in Section 2.2 and panel d shows the ion temperature measurements. The red bars in panels a–c are the $1-\sigma$ error bars. These error estimates are likely low, because the line-of-sight velocity measurements from several range gates were averaged in the vector velocity fit, but the potentially strong correlations between adjacent range gates in the long pulse experiment were neglected.

Around 20:43 UT and 21:58 UT, $V_\|$ shows upward field aligned motions of ions. Maximum upward velocity is observed around 21:58 UT. This further supports that there is an upward motion of plasma along magnetic field lines resulting in the uplift of the ionosphere with the arrival of the LSTIDs. If the ionosphere is raised to higher altitudes where recombination is lower during photoionization conditions, plasma can sustain there for a longer time resulting in the enhancement in electron density and TEC (Prölss, 1997; Schunk & Nagy, 1978; Volland, 1995). The terminator at 300 km altitude is at 00:20 UT for Millstone Hill, indicating that photoionization still prevails and can contribute to the increase in electron density. In an earlier case study of an extended
<figure><img src="image_12.png" /><figcaption>**Figure 9.** Electric field and field aligned velocity calculated from Millstone Hill radar ion velocity measurements on 14 March 16 UT–15 March 03 UT. (a) Northward electric field, (b) eastward electric field and (c) field aligned velocity, positive upwards, and (d) ion temperature from zenith measurements. The dashed vertical lines show the arrival time of the LSTIDs at Millstone Hill and error bars in red.</figcaption></figure>

positive storm phase driven by a moderate geomagnetic storm, Goncharenko et al. (2007) reported an increase in the peak height of the F-layer during the passage of TID and an associated increase in electron density. Also, Saito et al. (2002) reported an uplift of F-layer during the passage of a medium scale TID in the Japanese sector.

One of the possible mechanisms that could explain the uplift of the ionosphere during the passage of the TADs/TIDs is that these disturbances carry equatorward neutral winds along with them (Lu et al., 2008; Prölss, 1997) that can move the plasma up along the magnetic field lines leading to an ionospheric uplift. Another potential mechanism that leads to ionospheric uplift is an eastward electric field (Goncharenko et al., 2007; Huang et al., 2005; Mannucci et al., 2005).

Figure 9b shows the zonal electric field component with positive values indicating eastward electric field, which is derived from Millstone Hill measurements. During the observed positive storm phase and during the sudden uplift at 20:43 UT and 21:58 UT, the electric field is directed westward with small magnitude. Therefore, it is evident that the uplifts of the ionosphere are not driven by an eastward electric field. Also, the ion temperature, Figure 9d, shows no significant increases during the studied interval, indicating that ion heating due to strongly enhanced electric fields does not occur. This leads to the conclusion that the uplifts and the associated $N_e$ are potentially caused by the equatorward wind carried along with the LSTIDs.

It is observed in the keograms of DTEC in Figure 7, that with the propagation of the LSTIDs, there is a simultaneous expansion of TEC enhancement to lower latitudes. As these LSTIDs propagate to lower latitudes, they cause enhancements in TEC along their propagation paths. Goncharenko et al. (2007) reported a similar enhancement in TEC propagating to lower latitudes with the propagation of TIDs.

Followed by the sudden uplifts, downlifts of the F-region are observed. The downlift is more evident during the overpass of the second LSTID. The ionosphere started to descend, around 22:20 UT and continued until 23:10 UT (Figures 4d and 5d). During this period of descent, the vertical ion velocity is downward, see Figure 4f. The reversal in the direction of the vertical velocity is also seen in $V_\|$, shown in Figure 9c. After 22:20 UT, $V_\|$ mostly shows downward motion of ions, with relatively high magnitudes in Figure 9c. During this downward motion around 23 UT, a maximum in $N_e$ is observed with the peak height at 285 km, see Figures 4d and 5d (black curve). In Figure 4d we have calculated the TEC from the Millstone Hill electron density measurements upto an altitude of 800 km, which is overlaid on the density as a white curve, and reaches a maximum at 22:42 UT after uplift T2, during the downlift. The GNSS estimate of TEC shows a maximum at 22:30 UT and the peak in the DTEC at
Millstone Hill latitude and longitude occurs at 22:35 UT (Figure 7f), both during the downlift. Furthermore, the DTEC enhancement follows the second LSTID from high to low latitudes.

The downlift of the ionosphere could be caused either by a poleward wind or by a westward electric field associated with the TID/TAD. The electric field measurements from Millstone Hill radar (Figure 9c) show no significant westward electric field when the downlift is observed. This indicates that the downlift is plausibly caused by poleward wind associated with the LSTID. Shiokawa et al. (2007) reported a decrease in F-layer height following the passage of a TAD. They attributed the decrease in F-layer height to northward wind turning caused by the rarefaction after the passage of the TAD. Lu et al. (2008) reported a decrease in F-layer peak height caused by northward wind associated with TAD during a positive ionospheric storm.

During the downlift, we observe downward ion velocity in Figure 4f after 22:20 UT. The continuity equation along the vertical direction (z) can be written as

$$ \frac{dn_i}{dt} = q_i - l_i - \frac{d(n_i v_{iz})}{dz}, \qquad (3) $$

where $n_i$ is the ion density, $v_i$ is the ion velocity, $q_i$ is the ion production rate and $l_i$ is the ion loss rate. If the production and loss are roughly in balance, then the ion density is controlled by the vertical gradient in the ion density flux, as indicated by Equation 3. Therefore, the enhancement in peak electron density during the downlift could be caused by the vertical gradient in the number flux, which produces compression of plasma from above toward the peak altitude. Because the horizontal neutral winds affect the vertical ion motions, a gradient in vertical wind could be associated with spatial variation in the LSTID-driven poleward wind perturbation.

The effect of compression can possibly be seen in Figure 5d for the two electron density profiles at 22:42 UT and 22:57 UT (red and black curves). At 22:42 UT, the peak electron density is $9.1 \cdot 10^{11} \text{ m}^{-3}$ at about 300 km altitude. The measured electron density profile at 22:57 UT shows a downward shift of the peak height to 285 km and an increase in peak electron density to $9.5 \cdot 10^{11} \text{ m}^{-3}$. There are no additional zenith measurements between 22:42 and 22:57 UT because of the radar mode as explained in Section 2.2.

In Figure 10 we test Equation 3 for these two profiles by assuming that the production and loss balance and the vertical ion number flux $n_{i1}v_{i1}(z)$ measured at time $t_1 = 22:42$ UT stays constant until $t_2 = 22:57$ UT. Then, $n_{i2} = n_{i1} - (t_2 - t_1)(n_{i1}v_{i1}(z_2) - n_{i1}v_{i1}(z_1))/(z_2 - z_1)$. Figure 10a shows the ion density (black) and the vertical ion velocity (red) profiles measured by the radar at 22:42 UT. The vertical ion velocity is directed downward and exhibits vertical shear, which implies that there is vertical shear in the poleward neutral wind. The corresponding ion number flux profile is shown in Figure 10b. Figure 10c shows the vertical gradient of the ion flux, where the gradient is positive at altitudes above 340 km. However, the gradient is negative below 340 km. As Equation 3 shows, if the gradient is negative, it leads to an increase in ion density. The red curve in Figure 10d shows the estimated ion density profile at 22:57 UT, the blue curve shows the measured ion density profile at 22:57 UT and the black dashed curve shows the ion density profile at 22:42 UT. We can see that the estimated ion density increased from the preceding condition at heights below 340 km and decreased at heights above 340 km, as expected from the ion flux gradient profile. Comparing the estimated and measured profile at 22:57 UT, there is a good agreement between the two from 180 to 285 km, reproducing the measured peak ion density at 285 km. For two points above the measured peak ion density, the estimated densities show larger values than the measured ones, indicating that the assumption of stationary ion flux did not hold there. However, at other altitudes the match is relatively good, showing that the ion density profile was greatly influenced by the vertical gradient in ion number density flux and vertical compression played a role.

The peak value of TEC calculated from the Millstone Hill radar data is achieved at 22:42 UT, which is during the descent of the ionosphere during the second LSTID. Compression of the ionosphere alone cannot explain the TEC enhancement, but an additional mechanism is needed. Two possible mechanisms that can cause increases in TEC during the downlifts are downward plasma flux from the plasmasphere or increase in atomic oxygen due to storm-time changes in thermospheric circulation. (a) As the ionosphere is pushed down, the sudden downward flux can cause a loss of plasma in the topside ionosphere. This can cause plasmaspheric influx down into the topside ionosphere, which in turn could increase TEC (e.g., Li et al., 2018; Y. Cai et al., 2022). (b) Heating at high latitudes may cause the upwelling of the polar atmosphere and downwelling of the neutral species through constant pressure level at mid-latitudes, increasing atomic O density relative to molecular constituents (Buonsanto, 1999; Fuller-
<figure><img src="image_13.png" /><figcaption>**Figure 10.** The figure shows the estimation of electron density profile at 22:57 UT estimated from the profile at 22:42 UT, considering that the temporal evolution of the electron density profile is solely controlled by the electron density flux gradient in the vertical direction. (a) Measured electron density (black curve) and vertical ion velocity (red curve, positive upwards) profile at 22:42 UT, (b) electron density flux, (c) gradient in electron density and (d) electron density profile at 22:42 UT (black dashed curve), measured (blue) and estimated (red) electron density profiles at 22:57 UT.</figcaption></figure>
Rowell et al., 1994). The O atoms can be effectively ionized under sunlit conditions and thereby increase the ion density and TEC. Aa et al. (2021) reported a 10%–20% increase in O/N₂ ratio concurrent with an increase of 5–10 TECU during a positive ionospheric storm.

## 5. Conclusions

In this study, we analyzed the positive ionospheric storm phase on 14 March 2016 during a moderate geomagnetic storm driven by an HSS/SIR. The positive ionospheric storm phase appeared in the mid-latitude day-to-dusk sector during the initial and main phases of the geomagnetic storm. The increase in TEC showed a sharp poleward boundary coinciding with the equatorward edge of the sub-auroral region with SAPS. Spatial and temporal ionospheric variations at mid-latitudes (40°–60° MLAT) in the American sector were studied using the Millstone Hill incoherent scatter radar measurements and GNSS TEC. The high-latitude Joule heating was estimated using a novel method utilizing SuperDARN, AMPERE, and SuperMAG measurements and the results were used to identify the source regions of the LSTIDs that played a crucial role in producing the strong positive ionospheric storm phase.

Analysis of detrended TEC and Northern hemisphere Joule heating maps revealed that two distinct LSTIDS were launched from the auroral oval, one at about 19:50 UT and the other at about 21:20 UT. The first LSTID propagated southward with a velocity of 490 m/s and the second one south-westward with a velocity of 610 m/s. The second LSTID was associated with stronger Joule heating, higher amplitude, and more pronounced changes in the ionosphere.

The TEC increased at mid-latitudes simultaneously with the equatorward propagation of the LSTIDS. Before the arrival of the LSTIDS to Millstone Hill at 20:43 UT, the enhancement of DTEC (observed TEC - quiet day TEC) was 4 TECU, and after the passage of the two LSTIDS, the enhancement of DTEC had increased to 11 TECU, corresponding to 42% enhancement with respect to the quiet day level. Hence, most of the enhancement of TEC during the positive phase was associated with the passage of the LSTIDS.

The Millstone Hill radar measurements showed abrupt uplifts and downlifts of the F-layer followed by an enhancement in electron density during the overpasses of the LSTIDS. An ascend of about 55 km was observed simultaneously with a strong upward ion drift during the arrival of the second LSTID. Later, the ionosphere descended, during which the ion velocity reversed and the F-peak electron density reached a maximum value of $9.5 \times 10^{11} m^{-3}$. Close to this time, also the DTEC reached the maximum value. The electric field estimates from Millstone Hill measurements did not indicate the presence of an eastward electric field during the positive ionospheric storm phase, which rules out its contribution in causing the positive ionospheric storm.

The oscillation of the F-layer during the passage of the LSTIDS could plausibly be due to the neutral wind perturbations that propagated together with the LSTIDS. The LSTID troughs were related to uplifts of the F region and obviously with southward wind perturbations, whereas the LSTID crests were associated with downlifts of the F region and obviously with northward wind perturbations. The equatorward wind (poleward wind) can push the plasma up (down) along magnetic field lines. When the plasma is raised to higher altitudes under sunlit conditions, the reduced recombination rate, together with ongoing photoionization, results in both electron density and TEC enhancement. During the downlift of the ionosphere, we found observational evidence of plasma compression from above toward the peak-F altitude, increasing the F-peak density. To explain the increase in DTEC during the downlift, an additional mechanism is needed, which could be downward plasma flux from the plasmasphere or downward transport of atomic oxygen due to storm-induced changes in the thermospheric circulation. These processes in connection with the storm positive phase require further studies.

## Appendix A: Estimation of Joule Heating

Ignoring the neutral wind, the ionospheric Joule heating can be written as

$$Q_J = \vec{E} \cdot \vec{J} = |\vec{E}| |\vec{J}| \cos \gamma = \frac{|\vec{E}| |\vec{J}|}{\sqrt{1 + \alpha^2}}, \quad (A1)$$

where $\vec{E}$ is the convection electric field, $\vec{J}$ is the height-integrated horizontal current density, $\gamma$ is the angle between them, and $\alpha$ is the Hall-to-Pedersen conductance ratio. The last form is obtained by noting that according to the
<figure><img src="image_14.png" /><figcaption>**Figure A1.** An example showing the estimation of Joule heating: (a) electric field from SuperDARN, (b) divergence-free current density from SuperMAG, (c) curl-free current density from AMPERE, and (d) estimated Joule heating.</figcaption></figure>

ionospheric Ohm's law, $\cos \gamma = 1/\sqrt{1 + \alpha^2}$. The last form is used in our analysis, as it is numerically stable (no division by zero) and guaranteed to give positive Joule heating.

The convection electric field is obtained from the SuperDARN potential maps (Greenwald et al., 1995; Ruohoniemi & Baker, 1998; Shepherd & Ruohoniemi, 2000) by calculating the negative gradient of the electrostatic potential $\Phi$,

$$ \vec{E} = -\nabla\Phi. \qquad (A2) $$

An example of the electric field is shown in Figure A1 panel a. In the SuperDARN potential fitting spherical harmonic degree 8 and the statistical background model by Thomas and Shepherd (2018) are used. The height-integrated current density is calculated as a sum of the curl-free (CF) and divergence-free (DF) parts,

$$ \vec{J} = \vec{J}_{CF} + \vec{J}_{DF}, \qquad (A3) $$
Acknowledgments

This work was supported by the International Space Science Institute (ISSI) in Bern through ISSI International Team project #487, and by Kvantum Institute project Arctic Ionospheric Response to Space Weather Events, and by Research Council of Finland projects 348782 and 354521. A. Maute was supported by NASA award 80NSSC20K1784. Millstone Hill data were obtained through the Madrigal Database which is assembled and maintained by members of MIT Haystack Observatory Atmospheric Science Group. The authors thank Massachusetts Institute of Technology for providing the access and use of GNSS TEC data under support from the U.S. National Science Foundation (NSF) via Madrigal data base Grant AGS-AGS-195273. We acknowledge use of NASA/GSFC's Space Physics Data Facility's CDAWeb service and OMNI data for providing magnetic indices and solar wind parameters. We thank the AMPERE team and the AMPERE Science Data Center for providing data products derived from the Iridium Communications constellation, enabled by support from the National Science Foundation. We acknowledge the use of SuperDARN data. SuperDARN is a network of radars funded by national scientific funding agencies of Australia, Canada, China, France, Italy, Japan, Norway, South Africa, the United Kingdom, and the United States of America. For the ground magnetometer data in the SuperMAG database, the author gratefully acknowledges the following: Intermagnet; USGS, Jeffrey J. Love; CARISMA, PI Ian Mann; CANMOS; the S-RAMP Database, PI K. Yumoto and K. Shiokawa; the SPIDR database; AARI, PI Oleg Troshichev; the MACCS program, PI M. Engebretnson, Geomagnetism Unit of the Geological Survey of Canada; GIMA; MEASURE, PI Mark Moldwin; SAMBA, PI Eftyhia Zesta; 210 Chain, PI K. Yumoto; SAMNET, PI Farideh Honary; the IMAGE magnetometer network, PI L. Juusola; AUTUMN, PI Martin Connors; DTU Space, PI Anna Willer; South Pole and McMurdo Magnetometer, PIs Louis J. Lanzarotti and Alan T. Weatherwax; ICESTAR; RAPIDMAG; British Antarctic Survey; McMac, PI Peter Chi; BGS, PI Susan Macmillan; Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN); GFZ, PI Juergen Matzka; MFGI, PI B. Heilig; IGFPAS, PI J. Reda; University of L'Aquila, PI M. Vellante; BCMT, V. Lesur and A. Chambodut; data obtained in cooperation with Geoscience Australia, PI Marina Costelloe; AALPIP, co-PIs Bob Clauer and Michael Hartinger; SuperMAG, PI Jesper W. Gjerloev; Sodankylä Geophysical Observatory, PI Tero Raita; Polar Geophysical Institute, Alexander Yahnin and Yarolav Sakharov; Geological Survey of Sweden, Gerhard Schwartz; Swedish Institute of Space

as discussed below. The conductance ratio $\alpha$ needed in Equation A1 is estimated from the DF current using the empirical relations derived by Juusola et al. (2007):

$$ \alpha = \begin{cases} \frac{2.07}{\frac{36.54}{|\vec{J}_{DF}|} + 1} & \text{when } J_{DF,\phi} < 0, \\ \frac{1.73}{\frac{14.79}{|\vec{J}_{DF}|} + 1} & \text{otherwise.} \end{cases} \qquad (A4) $$

Here $J_{DF,\phi}$ is the eastward component of the DF current. The original equation given by Juusola et al. (2007) used $J_{DF,\phi}$ also in the denominator of the expression for calculating $\alpha$, but here we replace it with the magnitude of the DF current $|\vec{J}_{DF}|$ as advised by Dr Juusola (private communication).

The DF part of the current is obtained from the poloidal gridded ground magnetic disturbance $\Delta\vec{B}_P$ provided by SuperMAG (Waters et al., 2015) as

$$ \vec{J}_{DF} \approx -\frac{2}{\mu_0} \hat{e}_r \times \Delta\vec{B}_P. \qquad (A5) $$

This widely used approximation (see e.g., Section 2.2 in Untiedt & Baumjohann, 1993, and references therein) assumes an infinite, uniform current sheet in planar geometry, which would produce a constant magnetic field below and above it. This approximation is then applied at each point separately, so that horizontal variations are taken into account despite using a formula derived assuming a uniform current, see Figure A1b.

We use the gridded AMPERE magnetic field data product (Anderson et al., 2002; Waters et al., 2001, 2020) to estimate the CF part of the ionospheric current. The AMPERE project estimates the toroidal magnetic disturbance $\Delta\vec{B}_T$ associated with the FAC by fitting a vector spherical harmonic model to the magnetic field measured by Iridium communication satellites at about 780 km altitude. We use this to estimate the CF current as

$$ \vec{J}_{CF} \approx -\frac{1}{\mu_0} \left(\frac{r_{sat}}{r_I}\right)^{3/2} \hat{e}_r \times \Delta\vec{B}_T, \qquad (A6) $$

where $r_{sat}$ is the orbital radius of the Iridium satellites (at 780 km altitude) and $r_I$ is radius of the ionospheric E-layer (at 100 km altitude). An example plot is shown in Figure A1c.

Figure A1d represents the Joule heating rate, calculated using the method explained above.

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

Both the Millstone Hill incoherent scatter radar (Erickson, 2016) and the GNSS TEC (Coster, 2016) data used in this study are obtained from the Madrigal database. The TEC maps were projected in AACGM coordinate system using GeospaceLAB (L. Cai, 2025). Data for the solar wind, interplanetary magnetic field (IMF) and geomagnetic indices were taken from the OMNI data base (OMNI, 2025). For the Joule heating estimate, we used SuperDARN data (SuperDARN, 2021), AMPERE (AMPERE, 2022) and SuperMAG (SuperMAG, 2025). DMSP cross track ion velocity and auroral emission data are obtained from DMSP (2025).

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