Low Solar Flux Research Articles

Toroidal and poloidal magnetic fields at Venus. Venus Express observations

Magnetic field and plasma measurements carried out onboard Venus Express during solar minimum conditions suggest the existence of two kinds of magnetic field configuration in the Venusian ionosphere. We interpret these as the manifestation of two different types of generation mechanisms for the induced magnetosphere. A different magnetic field topology (toroidal and poloidal) arises if the induced currents are driven either by the solar wind motional electric field or by the Faraday electric field—a conducting ionosphere sees the magnetic field carried by solar wind as a time-varying field. At the dayside, both driving agents produce a similar draping pattern of the magnetic field. However, different magnetic field signatures inherent to both induction mechanisms appear at lower altitudes in the terminator region. The conditions at low solar EUV flux when the ionosphere of Venus becomes magnetized seem to be favorable to distinguish between two different types of the induced fields. We present cases of both types of the magnetic field topology. The cases when the effects of the Faraday induction become well noticeable are especially interesting since they provide us with an example of solar wind interaction with a tiny induced dipole field immersed into the ionosphere. Another interesting case when poloidal magnetic fields are evidently displayed is observed when the IMF vector is almost aligned with the solar wind velocity. In general case, both mechanisms of induction probably complement each other.

Modeling and observations of the low latitude ionosphere‐plasmasphere system at long deep solar minimum

The behavior of the low latitude ionosphere‐plasmasphere system in American‐Brazilian longitudes (30°W–120°W) in three seasons at the long deep solar minimum (2006–2010) is investigated using the theoretical model SUPIM and FORMOSAT‐3/COSMIC electron density and GIM‐TEC data. The data and model reveal some new aspects primarily for the low solar EUV fluxes. The ionosphere develops as a thin layer in the morning though becomes nearly as strong as that at normal solar minimum at around diurnal maximum (14 LT). However, after sunset the ionosphere‐plasmasphere system decays rapidly to become an extremely weak cold system prior to sunrise (05 LT) when the ionosphere contracts to about 1/3rd as strong as that at normal solar minimum, with peak density only about 1.8 × 105 cm−3, half width only about 150 km, and O+/H+ transition height as low as 475 km where the ion densities are only 104 cm−3 and ion temperatures are only 600 K. The mechanical effects of the neutral wind dominate over other processes, which causes the disappearance of the well known winter anomaly in TEC and Nmax, and lowest O+/H+ transition height (∼650 km at 14 LT and 475 km at 05 LT) occurring at around ±15° magnetic latitudes where the mechanical effects optimize. In addition, the ionosphere becomes weakest about 7 months after the solar activity dipped to the lowest level in 2008.

Enhanced lunar semidiurnal equatorial vertical plasma drifts during sudden stratospheric warmings

[1] Large scale electrodynamic and plasma density variations in the low latitude ionosphere have recently been associated with sudden stratospheric warming (SSW) events. We present average patterns of largely enhanced lunar semidiurnal equatorial vertical plasma drift perturbations during arctic winter low and high solar flux SSW events. These perturbations play a dominant role in the electrodynamic response of the low latitude ionosphere to SSWs. Our models indicate that the amplitudes of the enhanced lunar semidiurnal drifts are strongly local time and solar flux dependent, with largest values during early morning low solar flux SSW periods. These results suggest that ionospheric conductance strongly modulate low latitude ionospheric changes during SSWs. They also indicate that lunar semidiurnal effects need to be taken into account by global ionospheric models for their improved forecasting of the low latitude ionospheric response to SSW events, especially for low solar flux conditions.

C/NOFS satellite observations of equatorial ionospheric plasma structures supported by multiple ground-based diagnostics in October 2008

[1] In early October 2008, the C/NOFS satellite orbited near the magnetic equator at its perigee altitude of ∼400 km at dusk in the Peruvian sector. This provided an ideal opportunity for a comparison, under the current very low solar flux condition, of equatorial ionospheric disturbances observed with the Communication/Navigation Outage Forecasting System (C/NOFS) in situ measurements and ground-based observations available near Jicamarca Observatory. The primary objective was the comparison of plasma density disturbances measured by a Planar Langmuir Probe (PLP) instrument on the C/NOFS satellite with VHF scintillation activity at Ancon near Jicamarca for this period. Here we discuss in detail two extreme cases: one in which severe in situ disturbances were accompanied by mild scintillation on a particular day, namely, 10 October while there was little in situ disturbance with strong scintillation on 5 October. This apparent contradiction was diagnosed further by a latitudinal ground-based GPS network at Peruvian longitudes, a Digisonde, and the incoherent scatter radar (ISR) at Jicamarca. The crucial distinction was provided by the behavior of the equatorial ionization anomaly (EIA). The EIA was well-developed on the day having severe in situ disturbances (10 Oct). This led to lower equatorial plasma density and total electron content (TEC) at the equator and consequently reduced the scintillations detected at Ancon. On the other hand, on the day with severe scintillations (5 Oct), the EIA was not so well developed as on 10 October, leading to relatively higher equatorial plasma density and TEC. Consequently the severe scintillations at Ancon were likely caused by ionospheric structure located below the altitude of C/NOFS. The NRL SAMI2 model was utilized to gain a greater understanding of the role of neutral winds and electric fields in reproducing the TEC as a function of latitude for both classes of irregularities. Spectral studies with high resolution in situ PLP data were also performed. The power law spectra within the plasma bubbles showed two slopes: the low frequency slope being ∼−5/3 and the high frequency ∼−5 with a break around λ = 70 m. This particular type of two-slope spectra may be related to the extremely low solar activity and its impact on ion composition and temperature.

Forecast assessment of topside spread F at Jicamarca

The Jicamarca Unattended Long Term Investigations of the Ionosphere and Atmosphere (JULIA) radar operated from August 1996 through the present time, making a total of approximately 1500 nights of ionospheric irregularities associated with equatorial spread F (ESF) observations under different seasons, solar flux levels, and geomagnetic conditions. The climatology, persistence, and correlation with the phase of the Moon of premidnight and postmidnight irregularities are evaluated, and different forecasting tools are compared. During equinox, 1 day persistence information improves the forecast over climatology alone for most solar flux and geomagnetic activity conditions, with a significant improvement during periods at low solar flux. December solstice ESF observations show a high correlation with the lunar phase, with periods close to the full Moon presenting the highest probability of ESF occurrence. Using this information for geomagnetically quiet periods during December solstice, we further improve the forecast over the climatological model. For postmidnight and premidnight June solstice ESF, however, climatology still gives the best forecasting results.

Nighttime medium‐scale traveling ionospheric disturbances at low geomagnetic latitudes

Nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) are a commonly‐observed structure in the middle latitude ionosphere during the months surrounding the solstices and low solar flux conditions. MSTIDs consist of one or more narrow bands exhibiting uplift in F‐region electron density which are elongated from northwest to southeast in the northern hemisphere and southwest to northeast in the southern hemisphere. These electrified structures propagate westward and toward the equator in a direction perpendicular to their long dimension. Previously, this type of MSTID was not thought to occur at magnetic latitudes equatorward of the equatorial anomalies, or approximately ±15° geomagnetic latitude. Here, we present observational evidence from a field‐aligned narrowfield imaging system of MSTIDs existing very close to the magnetic equator. We discuss the implications of this observation in terms of the development and propagation of MSTIDs.

Comparing plasma bubble occurrence rates at CHAMP and GRACE altitudes during high and low solar activity

Abstract. Based on the multi-year data base (2001–2009) of CHAMP Planar Langmuir Probe (PLP) data and GRACE K-Band Ranging (KBR1B) data, typical features of ionospheric plasma irregularities are studied at the altitudes of CHAMP (300–400 km) and GRACE (~500 km). The phenomena we are focusing on are the equatorial plasma bubbles (EPBs). Similar seasonal/longitudinal (S/L) distributions of EPB have been found at both CHAMP and GRACE altitudes during solar active and quiet years. Peak EPB occurrence rates, defined as number of events within an S/L bin divided by the number of passes over that bin, decrease from the high and moderate solar flux period (2001–2005) to the low solar flux period (2005–2009) from 80% to 60% and 60% to 40% at CHAMP and GRACE altitudes, respectively. On average the occurrence rate increases linearly with solar flux at about the same rate at CHAMP and GRACE. For high flux levels (P10.7>200) non-linear increases are observed at GRACE. The occurrence rate increases rapidly after 19:00 local time (LT) during high solar flux periods. Around solar minimum rates increase more gently and peak around 22:00 LT. The highest occurrence rates are encountered at latitudes around 10° north and south of the dip equator. Results from the two altitudes support the notion that EPBs form regions of depleted plasma along geomagnetic fluxtubes. It is shown for the first time that in regions of high occurrence rates EPBs are associated with fluxtubes reaching greater apex heights than those in regions of low rates.

Thermospheric composition variations due to nonmigrating tides and their effect on ionosphere

The TIMED/GUVI thermospheric composition (O/N2 ratio) over 2009 reveals clear wave‐3 (summer season) and wave‐4 (Fall season) longitudinal dependence. The wave peak to valley varies from 7 to 11% of the background O/N2. The low Kp (yearly mean of 0.9) and low solar EUV flux indicates that the wave features were not caused by geomagnetic activity or solar EUV variation. A more likely explanation is nonmigrating diurnal eastward propagating DE2 and DE3 tides. Model calculations indicate that a 10% variation in O/N2 makes 1/3 contribution to the nightside equatorial 135.6 nm intensities compared to the variation due to the E‐region dynamo electric field. This study confirms that nonmigrating tides can penetrate toward the upper thermosphere, both O/N2 and the E‐region dynamo should be considered in analyzing ionospheric density variations due to nonmigrating tides. Comparison with MSIS results indicates that the nonmigrating tides have stronger effect on O/N2 than the migrating tides.

Variation in the ionospheric propagating factor M(3000)F2 at Ouagadougou, Burkina Faso

We use hourly monthly median values of propagation factor M(3000)F2 data observed at Ouagadougou Ionospheric Observatory (geographic12.4°N, 1.5°W; 5.9 o dip), Burkina Faso (West Africa) during the years Januar1987–December1988 (average F10.7 < 130 × 10 −22 W/m 2 /Hz, representative of low solar flux conditions) and for January 1989–December1990 (average F10.7 ⩾ 130 × 10 −22 W/m 2 /Hz, representative of high solar epoch) for magnetically quiet conditions to describe local time, seasonal and solar cycle variations of equatorial ionospheric propagation factor M(3000)F2 in the African region. We show that that seasonal trend between solar maximum and solar minimum curves display simple patterns for all seasons and exhibits reasonable disparity with root mean square error (RMSE) of about 0.31, 0.29 and 0.26 for December solstice, June solstice and equinox, respectively. Variability Σ defined by the percentage ratio of the absolute standard deviation to the mean indicates significant dissimilarity for the two solar flux levels. Solar maximum day (10–14 LT) and night (22–02 LT) values show considerable variations than the solar minimum day and night values. We compare our observations with those of the IRI 2007 to validate the prediction capacity of the empirical model. We find that the IRI model tends to underestimate and overestimate the observed values of M(3000)F2, in particular, during June solstice season. There are large discrepancies, mainly during high solar flux equinox and December solstice between dawn and local midnight. On the other hand, IRI provides a slightly better predictions for M(3000)F2 between 0900 and 1500 LT during equinox low and high solar activity and equinox high sunspot number. Our data are of great importance in the area of short-wave telecommunication and ionospheric modeling.

Morphology of the dayside ionosphere of Mars: Implications for ion outflows

Significant fluxes of tailward streaming ions have been detected in the Martian wake by instruments on spacecraft. Imposing outward fluxes at the top of a model will produce dayside ion density profiles that are characterized by smaller scale heights than those of diffusive equilibrium. We determine the maximum outward fluxes of ions, and those implied by radio occultation data, by constructing ∼180 models, with upward velocity boundary conditions in the range from 0 to (7–8) × 105 cm s−1 in small increments. As the upward velocity is increased, the topside ion or electron densities decrease until eventually the computed ion fluxes cease to increase, implying that this is the maximum outward flux that the ionosphere can sustain. By comparison to data, we derive a low solar activity upward flux of O2+ of ∼5 × 107 cm−2 s−1, and a maximum of ∼8 × 107 cm−2 s−1. For O+, the analogous fluxes are ∼4 × 106 cm−2 s−1 and ∼1.1 × 107 cm−2 s−1. We derive high solar activity upward fluxes of O2+ in the range ∼(1.2–1.6) × 108 cm−2 s−1, and a maximum of 2.4 × 108 cm−2 s−1. The O+ derived and maximum fluxes at high solar activity are ∼(1.5–2) × 107 and 5 × 107 cm−2 s−1, respectively. If these fluxes are averages over the dayside, we estimate total loss rates of O+ and O2+ of (2.8–11) × 1024 and (3.6–8.7) × 1025 s−1, respectively. Our computed escape rates of O+ are in substantial agreement with the models and data, but our O2+ escape rates are an order of magnitude larger. We discuss various mechanisms that would bring our O2+ escape rates or the O+/O2+ ratio into agreement with the measurements and models.

Solar and geomagnetic trends of equatorial evening and nighttime F region vertical ion drifts

F region vertical ion drifts were inferred from the evening and nighttime ionosonde data for two magnetic equatorial stations in West Africa: Ouagadougou (geographic: 12°N, 1.5°W; 5.9°N dip) and Ibadan (geographic: 7.9°N, 3.9°E; 6°S dip). We examine and discuss the short‐term patterns of behavior of ionospheric variability over Ouagadougou for 1986–1987 years of low solar activity (F10.7 = 80) and 1988–1989 years of high solar activity (F10.7 = 180) for quiet time, while that of Ibadan is for undisturbed (Kp ≤ 3.0) and disturbed (Kp &gt; 3.0) geomagnetic conditions during the 1958 International Geophysical Year (IGY) period, corresponding to high solar flux conditions (F10.7 = 208). Our results indicate that the evening and nighttime ion drift exhibits strong variations with the phase of the solar cycle but only small variations with geomagnetic activity. The characteristic values of evening prereversal velocity enhancements (PRE) vary between about 2–14 m/s and 12–22 m/s and 17–42 m/s and 18–40 m/s for low and high solar flux, unperturbed and perturbed conditions, in that order. The solar minimum evening reversal times are strongly season dependent, while the morning reversal times are season independent except during December solstice, which occurs earliest. During solar maximum, reversal times near dawn and dusk are essentially season independent except during June solstice season, which occurs late. The average occurrence time (1900 LT) of PRE is strongly independent of solar and magnetic variations apart from June solstice of high solar activity periods.

Seasonal variation of the longitudinal structure of the equatorial ionosphere: Does it reflect tidal influences from below?

We have examined the longitudinal structure of the equatorial ionosphere at 400‐km altitude in the noon and postsunset local time sectors in different seasons using 6 years of F‐region plasma density observations from the CHAMP satellite. A four‐peak wave structure is observed in both local time sectors. In the noon sector at a fixed solar flux level, this structure is observed to be most prominent around September equinox and weakest around December solstice. This seasonal dependence agrees well with that of the nonmigrating diurnal tides DE3, hence supporting a close coupling between the ionosphere and the mesosphere‐lower thermosphere possibly via the DE3 tidal modulation of the E‐layer dynamo. In the postsunset sector, however, such agreement cannot be claimed. In this sector, although the four‐peak structure can be observed in all seasons at moderate and high solar flux levels, its seasonal dependence does not follow that of the DE3 tides. This structure becomes indiscernible near solstices at low solar flux levels. Furthermore, the postsunset four‐peak wave structure exhibits larger amplitude than that during daytime, hence indicating that it is more likely an amplified feature rather than a remnant of the daytime structure. The prereversal enhancement is speculated to be a possible candidate to cause this amplification.

Analysis of near-surface ozone variations in Terra Nova Bay, Antarctica

AbstractOzone concentration measurements were made during December from 2001–2005 to quantify the contributions of different processes to near-surface ozone concentrations (O3) in Terra Nova Bay, Antarctica. The average O3 concentration was 20.3 ppbv. On days characterized by high solar radiation fluxes (HSR), significantly higher concentrations of O3 (21.3 ppbv) were recorded compared to days with low solar radiation fluxes (LSR days, 16.8 ppbv). High O3 concentrations could be related to strong winds from SW–NW. Three-dimensional back-trajectories show that air from the interior of the continent could affect O3 at Terra Nova Bay. Moreover, during HSR days, high O3 concentrations were also recorded in connection with weak circulation, suggesting that emissions from the Italian base (located 2 km north) could also represent a significant source of O3. To clarify the role of local pollution in Terra Nova Bay, O3 values were also calculated using the photochemical steady state (PSS) approximation under clear sky and cloudy conditions.

Statistical comparison of WINDII auroral green line emission rate with DMSP/SSJ4 electron energy input for high and low solar flux years

[1] The atmospheric response to precipitating auroral electrons under varying conditions of solar illumination is investigated using a statistical approach with respect to magnetic local time (MLT), solar zenith angle (χ), and geomagnetic latitude (MLAT) in the northern high-latitude region. Vertical profiles of daytime auroral O(1S) emission rate are obtained from UARS/WINDII measurements by removing the airglow components. These are compared with precipitating electron and proton input observed with DMSP-F10 and -F12 SSJ4 from which total energy and average energy are obtained. In order to investigate the solar flux effect, 2 a of data are examined, 1993 (high solar flux) and 1996 (low solar flux). Total electron energy flux increases with increasing solar zenith angle from 40° to 160° so that the energy flux during nighttime is much greater than in the daytime. During the daytime (χ ≤ 90°) the auroral atmospheric response correlates well with electron energy input yielding auroral production rates of 1.2 and 1.6 kR erg−1 cm2 s, but this relation breaks down beyond χ = 90°. It is suspected that nighttime thin sheet auroral profiles are not correctly inverted for this limb-viewing imager. A strong anticorrelation is found between the altitude of peak auroral volume emission rate and the average precipitating electron energy with slopes of 12.1 km keV−1 and 5.7 km keV−1 for 1993 and 1996, respectively. The dependence of auroral emission rate on solar illumination conditions is consistent with earlier work by Newell et al. (1996b) and others, but the clear dependence on solar zenith angle has not been demonstrated before.

Two gallium data sets, spin flavour precession and KamLAND

We reexamine the possibility of a time modulation of the low energy solar neutrino flux which is suggested by the average decrease of the Ga data in line with our previous arguments. We perform two separate fits to the solar neutrino data, one corresponding to ‘high’ and the other to ‘low’ Ga data, associated with low and high solar activity, respectively. We therefore consider an alternative to the conventional solar+KamLAND fitting, which allows one to explore the much wider range of the θ12 angle permitted by the KamLAND fitting alone. We find a solution with parameters Δm221 = 8.2 × 10−5 eV2, tan2θ = 0.31 in which the ‘high’ and the ‘low’ Ga rates lie far apart and are close to their central values and is of comparable quality to the global best fit, where these rates lie much closer to each other. This is an indication that the best fit in which all solar and KamLAND data are used is not a good measure of the separation of the two Ga data sets, as the information from the low energy neutrino modulation is dissimulated in the wealth of data. Furthermore, for the parameter set proposed one obtains an equally good fit to the KamLAND energy spectrum and an even better fit than the ‘conventional’ LMA one for the reactor antineutrino survival probability as measured by the KamLAND.

Zonal winds in the equatorial upper thermosphere: Decomposing the solar flux, geomagnetic activity, and seasonal dependencies

Using 3 years (2002–2004), over 16,400 orbits of measurements from the accelerometer on board the CHAMP satellite, we have studied the climatology of the equatorial zonal wind in the upper thermosphere. Several main features are noticed. The most prominent one is that the solar flux significantly influences both the daytime and nighttime winds. It overrides the geomagnetic activity effect, which is found to be rather limited to the nightside. An elevation of the solar flux level from F10.7 ≈ 100 × 10−22 W m−2 Hz−1 to F10.7 ≈ 190 × 10−22 W m−2 Hz−1 produces an eastward disturbance wind up to ∼110 m s−1. This consequently enhances the nighttime eastward wind but suppresses the daytime westward wind. A seasonal variation with weaker wind (by over 50 m s−1 at night) around June solstice than in other seasons has been observed regardless of solar flux and geomagnetic activity levels. The zonal wind is eastward throughout the night except around June solstice, where it ebbs to almost zero or turns even westward in the postmidnight sector at low solar flux level. The daytime wind is found to be generally more stable than the nighttime wind, particularly unresponsive to geomagnetic activities. Predictions from the Horizontal Wind Model find good agreement with the CHAMP‐observed wind at high solar flux levels during nighttime. At low solar flux levels, however, the model strongly underestimates the westward wind during morning hours by 50–120 m s−1 depending on season. The major difference between the HWM‐predicted and the CHAMP‐observed wind is seen in the phase of its diurnal variation. The CHAMP‐observed wind turns eastward around 1200–1300 MLT instead of 1600–1700 MLT predicted by the model. Comparisons with ground FPI observations and the NCAR Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) predictions show that the solar flux effect obtained from CHAMP is consistent with that modeled by TIEGCM. The solar flux dependence of zonal wind found here together with that of the zonal ion drift found in previous studies reflect the relative importance of the E‐ and F‐region wind dynamo in the thermosphere‐ionosphere coupling process. Furthermore, these wind measurements indicate that the Earth's atmosphere superrotates. The average superrotation speed amounts to about 22 m s−1 for a solar flux level of F10.7 ≈ 100 × 10−22 W m−2 Hz−1 but increases to 63 m s−1 for F10.7 ≈ 190 × 10−22 W m−2 Hz−1. Finally, the wind behavior presented in this study is longitudinally averaged and may differ from wind measurements at a certain longitude.

Response of peak electron densities in the martian ionosphere to day-to-day changes in solar flux due to solar rotation

Photochemical Chapman theory predicts that the square of peak electron density, N m , in the dayside ionosphere of Mars is proportional to the cosine of solar zenith angle. We use Mars Global Surveyor Radio Science profiles of electron density to demonstrate that this relationship is generally satisfied and that positive or negative residuals between observed and predicted values of N m 2 are caused by periods of relatively high or low solar flux, respectively. Understanding the response of the martian ionosphere to changes in solar flux requires simultaneous observations of the martian ionosphere and of solar flux at Mars, but solar flux measurements are only available at Earth. Since the Sun's output varies both in time and with solar latitude and longitude, solar flux at Mars is not simply related to solar flux at Earth by an inverse-square law. We hypothesize that, when corrected for differing distances from the Sun, solar fluxes at Mars and Earth are identical when shifted in time by the interval necessary for the Sun to rotate through the Earth–Sun–Mars angle. We perform four case studies that quantitatively compare time series of N m at Mars to time series of solar flux at Earth and find that our hypothesis is satisfied in the three of them that used ionospheric data from the northern hemisphere. We define a solar flux proxy at Mars based upon the E 10.7 proxy for solar flux at Earth and use our best case study to derive an equation that relates N m to this proxy. We discuss how the ionosphere of Mars can be used to infer the presence of solar active regions not facing the Earth. Our fourth case study uses ionospheric observations from the southern hemisphere at latitudes where there are strong crustal magnetic anomalies. These profiles do not have Chapman-like shapes, unlike those of the other three case studies. We split this set of measurements into two subsets, corresponding to whether or not they were made at longitudes with strong crustal magnetic anomalies. Neither subset shows N m responding to changes in solar flux in the manner that we observe in the three other case studies. We find many similarities in ionospheric responses to short-term and long-term changes in solar flux for Venus, Earth, and Mars. We consider the implications of our results for different parametric equations that have been published describing this response.

Behavior of the ionosphere and thermosphere subject to extreme solar cycle conditions

A 1‐D global average ionosphere and thermosphere (GAIT) model is used to examine the climatological behavior of the upper atmosphere, subject to both extremely low and high solar flux. These extremes are justified, in part, by the Maunder Minimum and Grand Maximum epochs described by J. A. Eddy, as well as other studies involving cosmogenic isotopes and Sun‐like stars. As the irradiance falls below normal solar minimum levels, the concentration of O+ decreases rapidly relative to the molecular ions, such that the ratio foF2/foF1 approaches unity. When subject to exceptionally high solar fluxes, the ionospheric peak electron density (NmF2) unexpectedly plateaus, remaining relatively constant even as the photon flux continues to increase. In both cases, the state of the underlying thermosphere, particularly the neutral gas temperature, is found to be largely responsible. Model trends are discussed in relation to ionospheric observations, specifically the preponderance of so‐called ionospheric G conditions at solar minimum and foF2 saturation at solar maximum, as well as the problem of Earth's global helium budget.

On the relation between solar wind, pseudobreakups, and substorms

A statistical study of pseudobreakups and substorms is performed using Polar UV images from a 3‐month period in winter 1998–1999. Data from the ACE solar wind monitor are examined in order to determine the influence of solar wind parameters on the occurrence of different substorm and pseudobreakup types. The results confirm that the IMF clock angle and the amount of solar wind energy flux control the strength of a substorm. The majority of large substorms appear when the IMF is strongly southward and the solar wind energy flux is high. Most small substorms occur during weakly positive or zero IMF Bz and low solar wind energy flux values. Pseudobreakups are associated with even lower energy fluxes than small substorms and appear typically for weakly positive IMF Bz. These results are in agreement with the scenario that pseudobreakups essentially are very weak substorms. Pseudobreakups appear during quiet times and during the growth phase or the recovery phase of weak or medium strong substorms. Time periods of enhanced geomagnetic activity with recurrent substorms are devoid of pseudobreakups. A detailed analysis of the different pseudobreakup types reveals that quiet time pseudobreakups appear predominantly during northward IMF. At least 20 percent of these appear at the poleward oval boundary. Optically, they do not differ much from very weak substorms. Growth phase pseudobreakups develop typically at the end of a 1 to 2 hour long excursion from northward to weakly southward IMF and are followed by quite weak substorms. A large majority of recovery phase pseudobreakups occur at a strongly polewardly displaced oval boundary at the end of a very active recovery phase. A considerable decrease of the polar cap size during the preceding substorm is connected to a northward turning of the IMF.

Average nighttime F region disturbance neutral winds measured by UARS WINDII: Initial results

We use low‐ and mid‐latitude wind data from the Wind Imaging Interferometer (WINDII) on board the Upper Atmosphere Research Satellite (UARS) to study the average response of nighttime upper thermospheric winds to geomagnetic activity. We calculate perturbation winds in magnetic coordinates and analyze them as a function of magnetic local time, latitude, geomagnetic activity, and solar EUV flux. The nighttime zonal disturbance winds are predominately westward, with the strongest effects extending from dusk at 70° to midnight at 45°. Westward disturbance winds are also observed throughout most of the night at low latitudes, where they change to eastward at dawn. Eastward perturbations occur in the post‐midnight sector above 50°. The meridional disturbance winds are primarily equatorward above 40° and after 0300 MLT. In the midnight sector during low and moderate solar flux conditions, poleward winds are observed below 40°; during solar maximum, the perturbations are largely equatorward throughout the night.