bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagation of ULF waves through the ionosphere: Analytic solutions for oblique magnetic fields
Authors: Sciffer M. D. , Waters C. L.
Published: November 2002

The propagation of ultra low frequency (ULF; 1‐100 mHz) waves from the magnetosphere to the ground is examined in the presence of oblique background magnetic fields. The problem is developed analytically for a thin sheet ionosphere, neutral atmosphere, and perfectly conducting ground. The cold plasma, ideal magnetohydrodynamic (MHD) Alfvén wave modes are assumed to propagate in the MHD medium above the ionosphere. A reflection and wave mode conversion coefficient matrix (RCM) is derived which describes mixing and conversion between shear Alfvén and fast mode energy when interacting with the ionosphere/atmosphere/ground system. The RCM is found to depend in a complicated way on the background magnetic field dip angle, the horizontal wave vector, and the conductivity of the ionosphere. For an oblique background magnetic field, in the XZ plane, the perpendicular wave number, ky, is shown to be a critical parameter that determines reflection and mode conversion characteristics. This study also highlights the need for spatial information of ULF wave energy in order to interpret experimental ULF wave data recorded at ground level in terms of magnetospheric processes.
Photoelectron flux and nightglow emissions of 5577 and 6300 Å due to solar wind electron precipitation in Martian atmosphere
Authors: Seth S. P. , Haider S. A. , Oyama K. I.
Published: November 2002

The detailed model calculations of photoelectron flux and nightglow emissions of 5577 and 6300 Å lines are made using analytical yield spectrum approach based on Monte Carlo method. The calculated photoelectron spectra are compared with electron reflectometer (ER) measurements made by Mars Global Surveyor (MGS) at the energy range 10–1000 eV. This calculation suggests that X‐ray ionization is an important process in the dayside ionosphere of Mars at energy greater than 90 eV, and 10‐ to 90‐eV electron population is controlled by photoionization and photoelectron impact ionization. The secondary electron flux, excitation rates, emission rates, and limb intensities of 5577 and 6300 Å lines are calculated due to precipitation of solar wind electrons observed by ER experiment at the energy range 10–1000 eV in the nighttime ionosphere of Mars. In the vicinity of the peak altitudes, it is found that excitation/emission of atomic oxygen after the electron impact dissociation of CO2 is the main source of 5577 Å emissions, while dissociative recombination of molecular oxygen ion is the major source of 6300 Å emissions after the electron impact excitation of atomic oxygen. The calculated secondary electron spectra are also compared with ER experiment measurements between energies of 10 and 1000 eV. The nightglow limb intensities are compared with the upper limit set by Mars 5 spectroscopic observations.
Low‐latitude solar wind during the Fall 1998 SOHO‐Ulysses quadrature
Authors: Poletto G. , Suess S. T. , Biesecker D. A. , Esser R. , Gloeckler G. , Ko Y.‐K. , Zurbuchen T. H.
Published: November 2002

The Fall 1998 Solar and Heliospheric Observatory (SOHO)‐Ulysses quadrature occurred when Ulysses was at 5.2 AU, 17.4°S of the equator, and off the west limb of the Sun. SOHO coronal observations, at heliocentric distances of a few solar radii, showed that the line through the solar center and Ulysses crossed, over the first days of observations, a dark, weakly emitting area and through the northern edge of a streamer complex during the second half of the quadrature campaign. Ulysses in situ observations showed this transition to correspond to a decrease from higher‐speed wind typical of coronal hole flow to low‐speed wind. Physical parameters of the low‐latitude coronal plasma sampled over the campaign are determined using constraints from what is the same plasma measured later in situ and simulating the intensities of the hydrogen Lyman‐α and OVI 1032 and 1037 Å lines, measured by the Ultraviolet Coronagraph Spectrometer on SOHO. It appears that low‐latitude wind from small coronal holes and polar wind have different characteristics in the corona, differences well known at interplanetary distances through in situ experiments. Small, low‐latitude coronal holes have a higher expansion factor than typical polar holes, and their plasma moves at a lower speed than plasma from polar holes, reaching, at 3.5 Rsun, only about one‐fifth of the terminal speed. Wind emanating from bright regions, above streamer complexes, is, at the altitudes we analyzed (i.e., 3.5 and 4.5 Rsun), about a factor 3 slower than the low‐latitude coronal hole wind, implying a shift to even higher altitudes of the region where plasma gets accelerated. We surmise that open field regions, interspersed amidst closed coronal loops/streamers, may be at least partially responsible for the well‐known slow wind speed variability. As in polar fast wind, OVI ions move faster than protons, over the range of altitudes we sampled, and are frozen‐in at temperatures of ≈1.3–1.5 106 K, depending on the site where the outflow originates. An oxygen abundance variation from a value of 8.55, in low‐latitude holes, to 8.73 in bright areas, has also been inferred.
Short‐term and long‐term variability of solar emissions in recent years
Authors: Kane R. P.
Published: November 2002

The short‐term (27‐day sequences) and long‐term variabilities of solar line emissions (120–280 nm) and radio emissions (245–2800 MHz) were studied during 1991–2000. The amplitudes of the 27‐day oscillations were obtained by Maximum Entropy Method (MEM) and Multiple Regression Analysis (MRA). The amplitudes were small (1–2%) near the solar surface and in the lower chromosphere, increased to ∼10% in the upper chromosphere and lower transition region, decreased in the upper transition region, increased to ∼12% in the lower corona, and decreased thereafter in the middle and upper corona. The long‐term variability also showed qualitatively a similar pattern. The pattern is similar to the dip around 100,000 K observed in the emission measure distribution in the solar atmosphere.
Ultraviolet insolation drives seasonal and diurnal space weather variations
Authors: Newell Patrick T. , Sotirelis Thomas , Skura Joseph P. , Meng Ching‐I. , Lyatsky Wladislav
Published: November 2002

We present several findings that improve the understanding of the seasonal and diurnal variation in auroral and magnetospheric activity. The total ionospheric conductivity in the nightside auroral oval from UV insolation (ΣP) is calculated, and its seasonal and diurnal variation is shown to correlate very highly with that of the Am and AL indices of geomagnetic activity (r = 0.89 and r = 0.75, respectively). Such excellent correlations with Am have been previously obtained by other researchers using instead the acute angle between the Earth's dipole axis and the Earth‐Sun line, ψ. However, the ionospheric conductivity formulation provides a more physical model to explain the equinoctial (McIntosh) effect. Namely, the level of geomagnetic activity is well‐ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither. We improve calculations of the expected pattern of seasonal and diurnal variations in the solar wind input. The elliptical nature of the Earth's orbit results in observed interplanetary magnetic field (IMF) strengths about 7% larger in January than June. When the Sun's spin axis tilt to the ecliptic plane is considered, the predicted IMF southward component (Bs) maximizes in February, as is observed. We also calculate the seasonal and diurnal variation of a more general solar wind–magnetosphere coupling function, EKL. EKL proves to have little (0.5%) diurnal variation and has a seasonal variation of about 14%. For the first time, the seasonal and diurnal variation in the ΦPC, the polar cap flux (from Polar UVI observations, cross‐calibrated to a DMSP‐based standard) and in magnetotail stretching (the b2i index) are presented. Magnetotail stretching proves to correlate better (r = −0.57) with EKL than with ΣP. ΦPC correlates better with ΣP, but the correlation (r = −0.49) is not nearly as strong as that for the indices of geomagnetic activity, Am and AL. Our survey of the seasonal and diurnal variation of the magnetosphere thus shows that some aspects (geomagnetic indices) correlate best with UV insolation, while others (magnetotail stretching) correlate best with solar wind input.
Solar flares, type III radio bursts, coronal mass ejections, and energetic particles
Authors: Cane H. V. , Erickson W. C. , Prestage N. P.
Published: November 2002

In this correlative study between >20 MeV solar proton events, coronal mass ejections (CMEs), flares, and radio bursts it is found that essentially all of the proton events are preceded by groups of type III bursts and all are preceded by CMEs. These type III bursts (that are a flare phenomenon) usually are long‐lasting, intense bursts seen in the low‐frequency observations made from space. They are caused by streams of electrons traveling from close to the solar surface out to 1 AU. In most events the type III emissions extend into, or originate at, the time when type II and type IV bursts are reported (some 5 to 10 minutes after the start of the associated soft X‐ray flare) and have starting frequencies in the 500 to ∼100 MHz range that often get lower as a function of time. These later type III emissions are often not reported by ground‐based observers, probably because of undue attention to type II bursts. It is suggested to call them type III‐l. Type III‐l bursts have previously been called shock accelerated (SA) events, but an examination of radio dynamic spectra over an extended frequency range shows that the type III‐l bursts usually start at frequencies above any type II burst that may be present. The bursts sometimes continue beyond the time when type II emission is seen and, furthermore, sometimes occur in the absence of any type II emission. Thus the causative electrons are unlikely to be shock accelerated and probably originate in the reconnection regions below fast CMEs. A search did not find any type III‐l bursts that were not associated with CMEs. The existence of low‐frequency type III bursts proves that open field lines extend from within 0.5 radius of the Sun into the interplanetary medium (the bursts start above 100 MHz, and such emission originates within 0.5 solar radius of the solar surface). Thus it is not valid to assume that only closed field lines exist in the flaring regions associated with CMEs and some interplanetary particles originating in such flare regions might be expected in all solar particle events.
Sunspot activity and the long‐term variation of the Sun's open magnetic flux
Authors: Wang Y.‐M. , Sheeley N. R.
Published: November 2002

The interplanetary magnetic field (IMF) originates in open magnetic regions of the Sun (coronal holes), which in turn form mainly through the emergence and dispersal of active region fields. The radial IMF strength is proportional to the total open flux Φopen, which can be estimated from source surface extrapolations of the measured photospheric field, after correction for magnetograph saturation effects. We derive the long‐term variation of Φopen during 1971–2000 and discuss its relation to sunspot activity. The average value of Φopen was ∼20–30% higher during 1976–1996 than during 1971–1976 and 1996–2000, with major peaks occurring in 1982 and 1991. Near sunspot minimum, most of the open flux resides in the large polar coronal holes, whereas at sunspot maximum it is rooted in relatively small, low‐latitude holes located near active regions and characterized by strong footpoint fields; since the decrease in the total area occupied by holes is offset by the increase in their average field strengths, Φopen remains roughly constant between activity minimum and maximum, unlike the total photospheric flux Φtot. The long‐term variation of Φopen approximately follows that of the Sun's total dipole strength, with a contribution from the magnetic quadrupole around sunspot maximum. Global fluctuations in sunspot activity lead to increases in the equatorial dipole strength and hence to enhancements in Φopen and the IMF strength lasting typically ∼1 year. We employ simulations to clarify the role of active region emergence and photospheric transport processes in the evolution of the open flux. Representing the initial field configuration by one or more bipolar magnetic regions (BMRs), we calculate its subsequent evolution under the influence of differential rotation, supergranular convection, and a poleward bulk flow. The initial value of Φopen is determined largely by the equatorial dipole strength, which in turn depends on the longitudinal phase relations between the BMRs. As the surface flow carries the BMR flux to higher latitudes, the equatorial dipole is annihilated on a timescale of ∼1 year by the combined effect of rotational shearing and turbulent diffusion. The remaining flux becomes concentrated around the poles, and Φopen approaches a limiting value that depends on the axisymmetric dipole strengths of the original BMRs. The polar coronal holes thus represent the long‐lived, axisymmetric remnant of the active regions that emerged earlier in the cycle.
Two solar cycles of nonincreasing magnetic flux
Authors: Arge C. N. , Hildner E. , Pizzo V. J. , Harvey J. W.
Published: November 2002

Since measurements began in the late nineteenth century, there has been a secular increase (with superposed ripples due to solar cycles) of the aa geomagnetic index [Mayaud, 1972]. Starting from this observation, Lockwood et al. [1999a, 1999b] conclude that the total open solar magnetic flux has increased by 41% from 1964 to 1995 and by 130% over all but the last 5 years of the twentieth century. However, solar data for more than two solar cycles ‐ Carrington maps from Mount Wilson, and Wilcox Solar Observatories and newly reanalyzed data from the National Solar Observatory ‐ show no secular trend in overall photospheric flux. More importantly, the magnetic flux open to interplanetary space (as calculated from photospheric measurements and assuming potential fields to a height of 2.5 R⊙) fails to show evidence of a secular increase over the last two solar cycles. Like Lockwood et al., we do not explicitly take account of transient events. Thus both data and calculations imply that the Sun's average coronal magnetic flux has not increased over the last two solar cycles. Analysis of simulations with the potential field source surface model shows that the interplanetary magnetic flux is not simply related to the erupted photospheric solar magnetic flux. Both results are in agreement with the findings of Wang et al. [2000]. The topology, rather than the strength, of the emergent solar magnetic field may be a major determinant of the interplanetary magnetic field experienced at Earth.
Secular change in geomagnetic indices and the solar open magnetic flux during the first half of the twentieth century
Authors: Cliver E. W. , Ling A. G.
Published: November 2002

We examined several long‐term geomagnetic indices (u, Ci, and Hm) to substantiate the secular increase in the aa index during the first half of the twentieth century. The long‐term increase in aa and other geomagnetic indices was accompanied by a corresponding rise in the envelope of the sunspot number (∼130% increase of cycle averages). We used a correlation between solar cycle averages of sunspot number and solar open magnetic flux for recent cycles to infer a 140 ± 80% increase in the open flux between ∼1900 and ∼1950, comparable to the ∼130% increase in this parameter during the twentieth century deduced by Lockwood et al. [1999] from solar wind measurements during the space age. While the uncertainty in our result is large, our method, which is not based on aa, provides independent support for a substantial increase in the open magnetic flux during the last century.
Long‐term trends in interplanetary magnetic field strength and solar wind structure during the twentieth century
Authors: Richardson I. G. , Cliver E. W. , Cane H. V.
Published: November 2002

Lockwood et al. [1999] have recently reported a ∼ 40% increase in the radial component of the interplanetary magnetic field (IMF) at Earth between 1964 and 1996. We argue that this increase does not constitute a secular trend but is largely the consequence of lower than average fields during solar cycle 20 (1964–1976) in comparison with surrounding cycles. For times after 1976 the average IMF strength has actually decreased slightly. Examination of the cosmic ray intensity, an indirect measure of the IMF strength, over the last five solar cycles (19–23) also indicates that cycle averages of the IMF strength have been relatively constant since ∼ 1954. We also consider the origin of the well‐documented increase in the geomagnetic aa index that occurred primarily during the first half of the twentieth century. We surmise that the coronal mass ejection (CME) rate for recent solar cycles was approximately twice as high as that for solar cycles 100 years ago. However, this change in the CME rate and the accompanying increase in 27‐day recurrent storm activity reported by others are unable to account completely for the increase in aa. Rather, the CMEs and recurrent high‐speed streams at the beginning of the twentieth century must have been embedded in a background of slow solar wind that was less geoeffective (having, for example, lower IMF strength and/or flow speed) than its modern counterpart.
Composition signatures in ion injections and its dependence on geomagnetic conditions
Authors: Fu S. Y. , Zong Q. G. , Fritz T. A. , Pu Z. Y. , Wilken B.
Published: November 2002

Ion composition in substorm injections are investigated in detail by using data obtained from the magnetospheric ion composition spectrometer (MICS) on board the Combined Release and Radiation Effects Satellite (CRRES). A total of 398 injection events are identified from 4 February to 10 October, 1991. It is found that the flux enhancements of O+ ions in the “with‐oxygen” injections (O+ ions occupy more than 20% of the total energy density) are attributed to the flux intensifications of high‐energy (E/q ≥ 100 keV/e) ions. In contrast, the flux of H+ and He++ ions in the “without‐oxygen” injections (O+ ions are less than 20% total energy density) are enhanced by the intensification of lower energy H+ and He++ (around 50 keV/e) ions. Statistical results also demonstrate that the abundance of O+ ions in an injection strongly depends on the geomagnetic activity. Without‐oxygen events take place most probably in relatively weak geomagnetic activity conditions (Dst index is around −50 nT and Kp is around 4). The averaged fractional energy density of O+ ions increases roughly linearly with Kp index. All injections are found to be rich of oxygen ions during the strong magnetic storms (Dst < −100 nT). The present study indicates that the ionospheric oxygen ions are energized more readily during the storm conditions. When the near‐Earth reconnection occurs and expands from the central plasma sheet to the plasma sheet boundary layer or even to the lobe region, much more ionosphere origin O+ ions could participate and be energized in the magnetospheric dynamic process. This leads to a with‐oxygen injection in the inner magnetosphere. As to the without‐oxygen injection, it seems that only preexisting oxygen ions in the near‐Earth plasma sheet are accelerated and become a part of the injected ions, thus a rather low abundance of O+ ions can be observed.
Propagation in the ionosphere of convection changes following a sharp interplanetary magnetic field By transition
Authors: Senior Catherine , Cerisier Jean‐Claude , Thorolfsson Adalbjorn , Lester Mark
Published: November 2002

The nature of the response of the ionospheric convection to changes in the interplanetary magnetic field (IMF), either quasi‐simultaneous over the polar cap and auroral region or propagating with the flow velocity from the noon sector, remains to be understood. We examine the changes in the ionospheric plasma convection associated with a negative to positive transition in the dawn‐dusk component (By) of the IMF. A special mode of operation of the SuperDARN HF radars has been used, which provided both a large field of view and high‐resolution convection data on three beams. Close to noon, the most characteristic response concerns the position in magnetic local time (MLT) of the merging gap, the ionospheric footprint of the magnetopause reconnection line, which moves longitudinally with a velocity of 1 km/s. In addition, modifications of the convection, which are indirectly related to this motion, are observed. We attribute them to the “quasi‐instantaneous” effect related to the incompressibility of the plasma. Far from noon, in the morning sector, the convection reacts with an intermediate delay, which is neither instantaneous nor compatible with the above propagation velocity. A timing of the transition, based on the IMP 8 and Geotail satellites, located on the dawn flank of the magnetosphere and at the nose of the magnetopause, respectively, indicates that the ionospheric perturbation originates at the impact site of the By transition close to 1300 MLT, with a maximum delay of 3 min after it hits the nose of the magnetopause.
Relative contribution of ionospheric conductivity and electric field to ionospheric current
Authors: Sugino Masahiko , Fujii Ryoichi , Nozawa Satonori , Buchert Stephan C. , Opgenoorth Hermann J. , Brekke Asgeir
Published: November 2002

Based on 10 years of European Incoherent Scatter (EISCAT) radar data, we have investigated the variation of ionospheric conductivities, electric fields, and currents over magnetic local time (MLT). Pedersen and Hall currents are generally enhanced especially on the nightside, and the MLT dependencies of these two types of ionospheric currents is similar, as previously found for field‐aligned currents and auroral electrojets, respectively. We readdress the question to what extent the conductivity or the electric field contributes to the ionospheric current in different periods of MLT. On average, higher Pedersen conductivities are seen at 2000–0800 MLT in comparison with other MLT intervals, and these conductivities crucially contribute to Pedersen currents over two MLT sectors, around midnight and in the late morning. However, when the Pedersen current is stronger, not only the Pedersen conductivity but also the electric field becomes higher statistically on the nightside. Hall conductivities are also higher at about 2000–0800 MLT, showing two maxima, around midnight and in the late morning, and they increase more strongly than Pedersen conductivities on a statistical basis. Thus the nightside electrojets are mainly due to high Hall conductivities.
Ionospheric coupling, especially between ionogram‐recorded spread‐F and sporadic‐E enhancements at an equatorial‐anomaly crest station, Chung‐Li
Authors: Bowman G. G. , Mortimer I. K.
Published: November 2002

The analysis of ionograms recorded at Chung‐Li, a station situated near the crest of the equatorial ionosphere anomaly, finds that there is coupling between the nighttime F2 and Es layers at the onset of spread‐F, as is the case in midlatitudes. The results were obtained using isolated patches of spread‐F occurrence of duration 2 hours or less and for sunspot‐minimum years. It is suggested that the medium‐scale traveling ionospheric disturbances (TIDs), which are responsible for the spread‐F occurrence, influence both the F2 and Es levels almost simultaneously, as their travel is consistent with the evidence from these analyses, as well as that already available for midlatitudes. Some comments are made on the possibility that these medium‐scale TIDs may be responsible directly for some of the small‐scale irregularities observed at these equatorial latitudes.
Ionospheric signatures of split reconnection X‐lines during conditions of IMF Bz < 0 and |By| ∼ |Bz|: Evidence for the antiparallel merging hypothesis
Authors: Chisham G. , Coleman I. J. , Freeman M. P. , Pinnock M. , Lester M.
Published: November 2002

We present measurements of the convection electric field in the dayside ionosphere during extended intervals when the interplanetary magnetic field (IMF) has a negative Bz component and a significant By component (|By| ∼ |Bz|). Under these conditions, distinct differences in the ionospheric projection of the magnetopause reconnection X‐line are predicted by the antiparallel and subsolar merging hypotheses. Close to solstice, in the winter hemisphere the antiparallel merging hypothesis predicts two ionospheric merging lines which are significantly separated either side of noon. This prediction is not expected from any subsolar merging hypothesis (e.g., component merging). Using the method of global convection mapping with line‐of‐sight velocity data from the SuperDARN HF radar network, we present evidence of two distinct merging regions on the dayside magnetopause under these IMF and seasonal conditions. The two merging regions manifest themselves in the ionosphere as two regions of convection flow with a strong poleward component separated by a region of convection flow with either a weak poleward or equatorward component corresponding to an adiaroic portion of the polar cap boundary. The spatiotemporal nature of the actual convection scenario can be complicated, but the predicted pattern represents a recurring feature of the convection electric field under these IMF and seasonal conditions. These observations lend support to the hypothesis that antiparallel merging is the dominant form of magnetopause reconnection during these steady state IMF intervals. This suggests that the magnetic shear between magnetosheath and magnetospheric magnetic field lines plays the major role in determining the location of reconnection on the dayside magnetopause.
Linear theory of the mirror instability in non‐Maxwellian space plasmas
Authors: Pokhotelov Oleg A. , Treumann Rudolf A. , Sagdeev Roald Z. , Balikhin Michael A. , Onishchenko Oleg G. , Pavlenko Vladimir P. , Sandberg Ingmar
Published: November 2002

A unified theory of the mirror instability in space plasmas is developed. In the standard quasi‐hydrodynamic approach, the most general mirror‐mode dispersion relation is derived and the growth rate of the mirror instability is obtained in terms of arbitrary electron and ion velocity distribution functions. Finite electron temperature effects and arbitrary electron temperature anisotropies are included. The new dispersion relation allows the treatment of more general space plasma equilibria such as the Dory–Guest–Harris (DGH) or Kennel–Ashour‐Abdalla (KA) loss cone equilibria, as well as distributions with power law velocity dependence that are modeled by the family of κ‐distributions. Under these conditions, we derive the general kinetic mirror instability growth rate including finite electron temperature effects. As for an example of equilibrium particle distribution, we analyze a large class of κ to suprathermal loss cone distributions in view of application to a variety of space plasmas like the solar wind, magnetosheath, ring current plasma, and the magnetospheres of other planets.
Parametric cross‐field current instability in solar coronal holes
Authors: Markovskii S. A. , Hollweg Joseph V.
Published: November 2002

We consider a parametric instability of fast and Alfvén waves with length scales of the order of the proton inertial length. The instability is driven by currents that are associated with these pump waves and that flow in the direction perpendicular to the background magnetic field. The initial pump waves generate secondary ion cyclotron waves with length scales below the proton gyroradius. The important property of the cross‐field current instability is that it can exist at relatively low amplitudes of the pump waves. As a result, it can be excited in the solar corona, where turbulent fluctuations are much smaller than in the distant solar wind. The instability of the pump wave provides an additional mechanism of wave damping compared to direct cyclotron damping. This mechanism starts to operate in the region of spatial scales where the direct cyclotron damping is weak. We show that the wave amplitudes, derived from an observed spectrum of density fluctuations, are sufficient to excite the cross‐field current instability and that the turbulent heating associated with the instability is fast enough to provide a heating mechanism for protons in the coronal holes.
Effect of time‐dependent coronal heating on the solar wind
Authors: Lie‐Svendsen Øystein , Hansteen Viggo H. , Leer Egil
Published: November 2002

We have used a higher‐order fluid model to study the effect of time‐dependent coronal heating of protons on the solar wind, both for the fast wind from rapidly expanding coronal holes and for slower winds from a radially expanding geometry. The model extends from the chromosphere to 1 AU in order to capture the coupling between the chromosphere and corona and therefore the supply of plasma to the solar wind. The protons are heated in the perpendicular direction (relative to the magnetic field), assuming a simple sinusoidal variation with time. With a short time period for the heating, less than about 3 hours, the oscillations are efficiently damped in the inner part of the solar wind, even when the amplitude of the change in heating rate is large, leading to essentially steady state solutions near Earth. Heating with a period of 10 hours or so leads to perturbations in the wind near Earth that are comparable to Ulysses observations. Heating with a period of order 100 hours leads to essentially a series of steady state solutions. The mass flux perturbations are in general larger than the perturbations in wind speed, and for heating of coronal holes with periods of order 30 hours the mass flux perturbations are amplified in the wind by the interaction between the fast and slow components of the wind. In this case even moderate changes in the heating rate in the corona can produce solar wind interaction regions with forward and reversed shocks and large mass flux perturbations near Earth.
Probabilistic models of the Jovian magnetopause and bow shock locations
Authors: Joy S. P. , Kivelson M. G. , Walker R. J. , Khurana K. K. , Russell C. T. , Ogino T.
Published: November 2002

New three‐dimensional models of the Jovian magnetopause and bow shock were derived by combining spacecraft observations with boundary characteristics inferred from a magnetohydrodynamic (MHD) simulation. The MHD simulation provides polynomial forms parameterized by solar wind dynamic pressure. Observations from Pioneer 10 and 11, Voyager 1 and 2, Ulysses, and Galileo were used to establish the probability that regions surrounding Jupiter fall inside or outside these boundary surfaces. The magnetopause location was found to have a bimodal probability distribution with the two most probable standoff distances at 63 RJ (σ = 4 RJ) and 92 RJ (σ = 6 RJ). The bow shock location distribution appears bimodal, but a single distribution function description cannot be ruled out at the 95% confidence level. The mean bow shock standoff distance is 84 RJ (σ = 16 RJ). Analysis of solar wind measurements near 5.2 AU (interplanetary magnetic field, dynamic pressure, and Alfvén Mach number) suggests that the bimodal distribution of boundary positions results at least in part from the bimodal distribution of solar wind parameters in the vicinity of Jupiter. The probability density distributions of these parameters within and between regions of disturbed solar wind, caused by corotating interaction regions and coronal mass ejections, are statistically distinct. Smaller variations were also observed in these parameters over the solar cycle.
Relative solar and auroral contribution to the polar F region: Implications for National Space Weather Program
Authors: David M. , Sojka J. J. , Schunk R. W. , Heelis R.
Published: November 2002

When plasma in the polar cap F region becomes highly structured, patches, irregularities, and scintillations of HF signals may be observed. The topic of this paper is not the mechanism for structuring or distributing the plasma but rather the source of the plasma. By understanding the plasma source we gain insight into the specification and forecasting of ionospheric structures and irregularities as required for space weather applications. The two major sources of polar cap F region plasma are the solar EUV radiation and the auroral precipitation. The region over which solar EUV production occurs is readily modeled. In contrast, the auroral precipitation is not subject to diurnal or seasonal dependences in the same predictable manner; the auroral precipitation can almost be viewed as stochastic within certain geomagnetic coordinate constraints. In this study we use a physical model to separate the effects of solar EUV and auroral precipitation. We find that the auroral contribution does provide a far‐from‐negligible “baseline” level of polar cap F region plasma, upon which is superimposed the UT and seasonally dependent TOI. This baseline level of ionization is very difficult to predict or forecast since it is determined by plasma flux tube histories through extended regions of the auroral oval over several hours. This result raises the need for more advanced auroral precipitation modeling in order to obtain improved space weather specification. The inclusion of soft auroral precipitation is especially important since it can be a significant source of F region plasma.
Transpolar potential saturation: Roles of region 1 current system and solar wind ram pressure
Authors: Siscoe G. L. , Crooker N. U. , Siebert K. D.
Published: November 2002

MHD simulations give about the same dependence of transpolar potential on solar wind electric field (IEF) as the Hill model of transpolar potential, including saturation and dependence on ram pressure. In the Hill model, feedback of the region 1 current system is presumed to limit the rate of reconnection at the magnetopause thereby causing transpolar potential saturation. MHD simulations add as relevant information that in the saturation domain the region 1 current system usurps the role of the Chapman‐Ferraro current system, which disappears. This means that the region 1 current system takes on the role of providing the current and generating most of the magnetic field in the J × B force at the magnetopause that balances solar wind ram pressure. Viewed from this perspective, transpolar potential saturation results not from the region 1 current system limiting the rate of reconnection at the magnetopause but instead from ram pressure (more accurately, total solar wind stresses) limiting the total amount of current that can flow in the region 1 current system. Transpolar potential saturation is then the limit on transpolar potential that corresponds to the ram‐pressure limit on total region 1 current.
Examination of the storm/substorm relationship using global auroral X‐ray images
Authors: Anderson P. C. , Chen M. W.
Published: November 2002

Global measurements by the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) instrument on board NASA's Polar spacecraft of the auroral X‐ray emissions associated with substorms show significant differences between isolated substorms and those occurring during geomagnetic storms. During isolated substorms at substorm onset the emissions are initially seen in the premidnight‐to‐midnight sector and then spread toward dawn. The emissions extend beyond 12 magnetic local time (MLT) with the most intense emissions occurring in the postdawn sector. The timescales for the appearance of X‐ray emissions in the various morningside MLT sectors are consistent with the drift times of electrons, of energies that produce the photons observed by PIXIE, under the influence of a magnetospheric electric field inferred from ionospheric ion drift measurements. During substorm recovery, emissions die away gradually at increasing MLT values with the last of the emissions in the noon sector disappearing several hours (∼3–4) after substorm onset. During the stormtime substorms, onset‐associated emissions are seen in the premidnight‐to‐midnight regions and spread toward dawn as in the isolated substorms. However, the emissions do not reach much beyond ∼9–10 MLT and very intense emissions are seen in the predawn MLT sectors. These intense predawn emissions are seen throughout the stormtime periods and are associated with significantly enhanced magnetospheric convection. There are brief reductions in intensity in the morningside emissions shortly after substorm onset consistent with a brief reduction in the cross‐tail electric field, followed by intensification again in the predawn sector on a timescale consonant with drifting electrons under the influence of a magnetospheric electric field. We conclude that the differences between the temporal evolution and morphology of the auroral X‐ray emissions during isolated and stormtime subtorms are the result of pitch angle scattering mechanisms whose MLT distribution and intensity are dependent on the strength of the magnetospheric electric field.
Solar mean magnetic field variability: A wavelet approach to Wilcox Solar Observatory and SOHO/Michelson Doppler Imager observations
Authors: Boberg Fredrik , Lundstedt Henrik , Hoeksema J. Todd , Scherrer Philip H. , Liu Wei
Published: November 2002

Solar mean magnetic field (SMMF) measurements from the Wilcox Solar Observatory and with the SOHO/MDI instrument are described and analyzed. Even though two completely different methods of observation are used, the two data sets obtained show a strong similarity. Using continuous wavelet transforms, SMMF variability is found at a number of temporal scales. Detected SMMF signals with a 1–2 year period are considered to be linked to variations in the internal rotation of the Sun. Intermediate SMMF oscillations with a period of 80–200 days are probably connected to the evolution of large active regions. We also find evidence for 90 min variations with coronal mass ejections as a suggested origin.
Improving the real‐time ionospheric determination from GPS sites at very long distances over the equator
Authors: Hernández‐Pajares M. , Juan J. M. , Sanz J. , Colombo O. L.
Published: November 2002

As the authors have shown in previous work, the tomographic approach reduces significantly the mismodeling in the electron content determination using as data the ionosphere‐crossing radio signals of global navigation satellite systems (GNSS) such as the Global Positioning System (GPS). We present in this paper the results of a comprehensive study on the performance of an improved tomographic model of the ionosphere. This model is obtained from the GPS L1 and L2 carrier‐phase data, and it is used to estimate undifferenced and double‐differenced ionospheric corrections in real time and at very long distances between receivers (500–3000 km). The key point of the strategy is the combination of real‐time geodetic and ionospheric techniques to achieve a significant improvement in the reliability of carrier‐phase ambiguity resolution. This new approach includes also the use of smoothed pseudo‐ranges to help in ambiguity resolution. It has been tested under difficult ionospheric conditions, during four consecutive weeks in March–April 2001, at Solar Maximum, and at latitudes ranging from −40 to +40 degrees, so as to include the equatorial region. Dual‐frequency satellite altimetry from TOPEX‐Poseidon has been used to verify the accuracy of the ionospheric model over the oceans in that region.
Outstanding issues in the theory of radar aurora: Evidence from the frequency dependence of spectral characteristics
Authors: Moorcroft D. R.
Published: November 2002

A review of the spectral characteristics of radar aurora with a focus on the behavior over a wide range of frequencies gives a different perspective on several issues concerning auroral backscatter. Some of the conclusions from this study are as follows: (1) It appears most likely (but has certainly not been proven) that auroral backscatter at large geometric magnetic aspect angles has the same general characteristics at all frequencies; VHF observations showing other characteristics are most likely refracted to small aspect angle by sporadic E. (2) The broadest spectra are observed at neither HF or UHF, but at VHF frequencies, but it is unclear whether this is a topic for theoreticians to consider, or if it is an observational effect. (3) The flow angle dependence of sub‐ion‐acoustic UHF echoes and large aspect angle echoes both present theoretical challenges which remain unaddressed. (4) In spite of several attempts, there appears to be no satisfactory theory of either type 3 or type 4 echoes, especially when the frequency dependence of these echoes is considered.
Electron dynamics in the current disruption region
Authors: Ohtani S. , Takahashi K. , Lui A. T. Y.
Published: November 2002

Turbulent magnetic field observed at substorm onsets in the near‐Earth magnetotail is often regarded as a manifestation of the disruption of the local tail current. The present study examines the associated dynamics of electrons using data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer. Two well‐known events were selected, which took place on 28 August 1986 and 1 June 1985. The results are summarized as follows: (1) during the early period of the 28 August 1986 event, the average perpendicular energy changed very little even though the total magnetic field strength (BT) fluctuated by an order of magnitude; (2) except the period of (1), tends to change in proportion to BT in both events; and (3) and the density N tend to be anticorrelated, whereas such a tendency cannot be found for the parallel average energy. The successive trigger of the Ion Weibel instability (IWI) and the modified two‐stream instability (MTSI) is a possible explanation for (1). As the plasma sheet thins, the IWI may be triggered first, followed by the MTSI, which becomes unstable at higher frequencies than the IWI and energizes electrons. The observed sequence of magnetic turbulence and electron energization in the 28 August event is consistent with this idea. On the other hand, (1) can also be explained as far as magnetic turbulence is localized near the neutral sheet irrespective of its trigger mechanism. In this case electrons stay mostly outside of the turbulent region, and accordingly the variations of the average electron energy do not have to be correlated with the variations of magnetic field in the turbulence region. In contrast, when the source current of the magnetic turbulence is not located closely (compared with its characteristic scale), the resultant magnetic disturbance is more coherent and is distributed widely along the flux tube. In this case the observed magnetic field can represent the magnetic variation that electrons actually undergo, which explains (2). Result (3) indicates that for the full understanding, the electron dynamics need to be addressed in a three‐dimensional configuration. As the present study is based on two events, it remains to be understood how common the observed features are among current disruption events. However, the results of the present study as well as their implications for the magnetic turbulence, such as its confinement near the neutral sheet, provide additional information for further understanding of the current disruption process.
Normal fault control on bedrock channel incision and sediment supply: Insights from numerical modeling
Authors: Hardy Stuart , Gawthorpe Rob
Published: November 2002

The evolution of footwall drainage basins, sediment supply and hanging wall stratigraphy in rift basins is investigated using a coupled numerical model of detachment‐limited stream network evolution and coarse‐grained fan delta deposition. The response of bedrock channel networks to repeated footwall uplift events leads to increasing sediment supply over tens of thousands of years as the networks initiate and then expand, followed by relatively constant sediment supply as the stream networks reach their maximum spatial extent. The distinct time lag between the onset of tectonic activity and attainment of maximum sediment supply leads to stratigraphies that are initially aggradational and then progradational. Much of the variation in sediment supply is the product of the complex response and reorganization of the stream networks to faulting‐related perturbations and is not directly related to specific faulting events. Increasing fault displacement drives the system toward a new steady state with a higher sediment supply but is initially expressed as a retrogradation as sediment supply lags behind increased displacement. The converse is true for a decrease in fault displacement, which is initially expressed as progradation. Continual stochastic variation in recurrence interval leads to a system that does not achieve a steady state and as a result exhibits continual variation in sediment supply. The model results emphasize the local tectonic controls on stratigraphic architectures developed within extensional basins and the complex manner in which stream network growth and response to tectonic activity can be expressed in the stratigraphic record.
A two‐dimensional inverse model for extensional sedimentary basins 1. Theory
Authors: White Nicky , Bellingham Paul
Published: November 2002

A simple two‐dimensional inverse model for extracting the spatial and temporal variation of strain rate from extensional sedimentary basins is described. Our starting point is a forward model which allows basin stratigraphy to be calculated from any given strain rate distribution. This forward model includes potentially important effects such as flexural rigidity and the two‐dimensional conduction/advection of heat. Conversely, inverse modeling determines strain rate variation by minimizing the misfit between predicted and observed patterns of basin subsidence. No a priori assumptions about the number, duration, or intensity of rifting episodes are necessary. Instead, strain rate is allowed to vary smoothly throughout time and space. An important restriction is that vertical lines remain vertical, but this restriction can be relaxed if deemed necessary. We have successfully inverted different synthetic sedimentary basin geometries which were generated by forward modeling. Sensitivity analysis and noise tests demonstrate that strain rate patterns can be recovered with confidence. The relationship between stratigraphic misfit and elastic thickness shows that usually an upper limit, but not a lower limit, for elastic thickness can be retrieved. Our general approach is applicable to any extensional sedimentary basin, including highly extended passive margins, and it should be regarded as the stepping stone to more sophisticated two‐dimensional and three‐dimensional inverse modeling. In the companion paper, we apply our algorithm to a set of Phanerozoic extensional sedimentary basins located worldwide.