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# Will nanodust reappear in STEREO/WAVES data?

N Meyer-Vernet

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N Meyer-Vernet. Will nanodust reappear in STEREO/WAVES data?. Research Notes of the AAS, 2025, 9 (1), pp.17. 10.3847/2515-5172/adaf25 . obspm-04919838v2

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# Will Nanodust Reappear in STEREO/WAVES Data?

To cite this article: Nicole Meyer-Vernet 2025 *Res. Notes AAS* 9 17

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DRAFT VERSION JANUARY 29, 2025

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# Will nanodust reappear in STEREO/WAVES data?

Nicole Meyer-Vernet¹

¹LIRA, Observatoire de Paris, Université PSL, Sorbonne Université,
Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France

## ABSTRACT

Nanodust particles produced near the Sun by collisional breakup of larger grains are accelerated in the magnetised solar wind and reach high speeds outwards of 1 AU. Vaporisation and ionisation of fast dust grains impacting a spacecraft produce voltage pulses on wave instruments that enable them to act as dust detectors. Wave instruments on STEREO and on Cassini during its cruise phase detected a highly variable flux of fast nanodust. Both detections took place when the orientation of the solar magnetic dipole produced an interplanetary electric field that focused nanoparticles towards the heliospheric current sheet (HCS) - a geometry that is recurring because of the periodicity of solar activity.

## INTRODUCTION

Our solar system zodiacal cloud includes dust particles of various sizes. Since the Lorentz force acceleration on a particle of radius $r$ is proportional to the charge-to-mass ratio $q/m$ with $q \propto r$ (Mann et al. 2014) and $m \propto r^3$, it controls the dynamics of small particles. Nanodust gyroradii being much smaller than the scale of variation of the magnetic field $B$, they are picked up by the solar wind (of velocity $V$), gyrate around field lines and move at

$$v_D = \frac{(-V \times B) \times B}{B^2}$$

$(E \times B)$ drift) which exceeds 300 km/s farther than 1 AU, plus a smaller vertical drift $v_\theta \propto m$ (Northrop 1963). Nanoparticles created near the Sun can be trapped within 0.2 AU and released intermittently when the outer boundary of the trapping region changes due to transient magnetic field changes (Czechowski & Mann 2012).

Fast dust grains impacting a spacecraft vaporise and ionise, producing a released electric charge scaling linearly with mass but greater than the impact speed cubed. Charge recollection and induced voltages on the spacecraft and antennas produce voltage pulses on wave instruments, whose detection enables them to measure dust. Such a method, pioneered when Voyager 2 crossed Saturn's G ring (Aubier et al. 1983; Gurnett et al. 1983), has been widely applied and is currently used in the inner heliosphere onboard Parker Solar Probe (Page et al. 2020) and Solar Orbiter (Zaslavsky et al. 2021).

## INTERPLANETARY NANODUST OBSERVATIONS ON STEREO AND CASSINI

A nanodust particle impacting a spacecraft at 300 km/s produces a charge nearly as large as that produced by a much slower $\beta$ dust particle. STEREO/WAVES near Earth's orbit measured nanodust with a variable flux much higher than that of larger particles (Meyer-Vernet et al. 2009b; Zaslavsky et al. 2012; Le Chat et al. 2013). Nanodust was also detected on Cassini/RPWS when the instrument was episodically turned on during its cruise phase; the flux was highly variable (Schippers et al. 2014) and decreased as the inverse squared heliocentric distance (Schippers et al. 2015). Cassini observations during corotating fast and slow solar wind streams (CIR) confirmed that nanodust particles move at speeds near $v_D$ (Meyer-Vernet et al. 2016). Cassini/RPWS measured also jovian nanodust streams, in agreement with dedicated dust detectors (Meyer-Vernet et al. 2009a).

Corresponding author: Nicole Meyer-Vernet

nicole.meyer@obspm.fr
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Top. Polar solar magnetic field strength (in Gauss), from the Wilcox Solar Observatory (+ North: solid blue, - South: dashed red, average: medium solid, smoothed average: heavy solid). Negative (positive) north magnetic field produce a focusing (defocusing) electric field for positive charges. Both STEREO and Cassini nanodust observations took place during a focusing configuration. Bottom. Maximal tilt angle of the HCS (solid: classic PFSS model, dashed: radial boundary condition model).</figcaption></figure>

Nanodust was much easier to detect on Cassini than on STEREO because the power spectra are proportional to the impact rate whereas Cassini spacecraft projected surface area exceeded that of STEREO by one order of magnitude. Furthermore, the waveform measurement on Cassini had a higher sensitivity than on STEREO.

The high variability of the interplanetary nanodust flux observed by STEREO and Cassini was attributed to time-dependent production, transient magnetic field changes, and to the complex dynamics in presence of the HCS (Czechowski & Mann 2012; Juhasz & Horanyi 2013; Poppe & Lee 2022). A variable production rate was confirmed by PSP dust measurements (Szalay et al. 2021); furthermore, even if nanodust was uniformly distributed near the Sun, dynamics would produce a highly nonuniform flux near Earth's orbit.

An important parameter governing nanodust detectability by STEREO and Cassini is the polarity of the solar magnetic dipole (Meyer-Vernet 2007), which determines whether the interplanetary electric field is focusing or defocusing (Juhasz & Horanyi 2013; Poppe & Lee 2022). A defocusing electric field points away from the HCS, driving positively charged nanodust away from it; larger particles, having a faster vertical drift, quickly drift away; hence, only the smallest particles can go close to it and be observed near the ecliptic. In contrast, in focusing conditions, the larger nanograins tend to move near the HCS, enabling a broader size range to be detected when the HCS is weakly tilted and thus close to the ecliptic.

Figure 1 shows the periods of nanodust observation on STEREO and Cassini. Both occured during focusing conditions.

However, the periods of nanodust observation on Cassini (from 01:1999 to 03:2000) were determined by the instrument itself, which was turned on only episodically; furthermore, beyond the asteroid belt, the integration time was considerably reduced in order to measure radioemissions at a high rate, making the instrument unable to detect dust (Schippers et al. 2015). Cassini nanodust detection took place near solar activity maximum, so the HCS was generally far from ecliptic (see Fig. 1), which explains that the detected particles were smaller than 10 nm (see Fig 2
in Meyer-Vernet et al. (2016)), since these small particles have a smaller vertical drift (Juhasz & Horanyi 2013; Poppe & Lee 2022).

Interplanetary nanodust observations on STEREO started at the beginning of the mission and ended before the end of the focusing period, when the tilt angle of the HCS became large (see Fig.1). This indicates that, as suggested by Poppe & Lee (2022), STEREO A could only measure the largest nano grains (> 10 nm), which are most sensitive to it and were far from ecliptic when the HCS tilt was large. STEREO measuring larger nanograins than Cassini is coherent with the already mentioned higher sensitivity of Cassini for nanodust detection.

## CONCLUDING REMARKS

Cassini observed nanodust when the HCS was highly tilted, but it observed small nanodust particles, which are weakly affected by the vertical drift. In contrast, STEREO observed nanodust when the focusing interplanetary electric field pushed large nano particles towards the HCS, but the detection stopped when the HCS tilt angle increased too much. This indicates that nanodust may reappear in STEREO wave data, as suggested by Poppe & Lee (2022), when the HCS tilt is not too high.

Parker Solar Probe and Solar Orbiter orbit too close to the Sun for the speed of nanodust to be large enough to enable its detection by wave instruments, except when transients, as coronal mass ejections, would accelerate it sufficiently (Le Chat et al. 2015; O'Brien 2018). Such events would open up the possibility of nanodust detection at small heliocentric distances.

1. The magnetic field data were obtained at http://wso.stanford.edu/.

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