Solar Wind Dynamic Pressure Enhancements Research Articles

Double dayside detached auroras: TIMED/GUVI observations

On August 19, 2003, TIMED/GUVI detected, for the first time, two separated (double) dayside detached auroras (D‐DDA) simultaneously in all the GUVI channels, 121.6 nm, 135.6 nm, LBHS, and LBHL. The D‐DDA was located at magnetic latitudes around −57° and −63° (L shell 3.4 and 4.9) in the 0900–1030 MLT region. The high‐latitude part of the D‐DDA is much brighter than the low‐latitude part. DMSP particle observation indicates that the source of the D‐DDA is energetic (∼10 keV) precipitating protons/electrons from the ring current due to the solar wind dynamic pressure enhancements [Zhang et al., 2002] and/or northward turning of the IMF Bz. Observations of the D‐DDA provide the first single‐event evidence of multiple peaks in the statistical distribution of proton fluxes at energetic around and below 10 keV at low L‐shells [Milillo et al., 2001].

Sawtooth oscillations directly driven by solar wind dynamic pressure enhancements

We have examined four well‐defined events of sawtooth oscillations in energetic particle flux and magnetic field at geosynchronous orbit. During all four events, nearly simultaneous energetic particle flux enhancements and magnetic field variations occurred at all MLTs for each sawtooth cycle. Geomagnetic H component data at low to middle latitude also show a global H increase simultaneously with the geosynchronous responses at all MLTs, and the northern and southern PC indices generally show increases at each sawtooth cycle. All these are what is expected if solar wind pressure enhancements impacted the magnetosphere at times appropriate to have caused the onset of each sawtooth cycle. By directly checking the solar wind data, we find that there indeed exists a series of solar wind dynamic pressure enhancements for each sawtooth event. In identifying these pressure enhancements, we have found that the relative change in the dynamic pressure is important, particularly when the magnitude of the dynamic pressure is small and that even a modest dynamic pressure enhancement can result in significant changes in the magnetosphere when the IMF stays strongly southward for a long interval. We show that each cycle of the sawtooth oscillation can be reasonably associated in timing with a corresponding solar wind dynamic pressure enhancement. On the basis of this association and the global, simultaneous geosynchronous and ground responses, we suggest that the sawtooth oscillations studied in this paper are directly driven by series of solar wind pressure enhancements and are not a repetitive internal magnetospheric response to sustained enhanced solar wind energy input.

Geosynchronous magnetic field response to solar wind dynamic pressure pulse

We present a study of the response of the geosynchronous magnetic field to abrupt enhancements of the solar wind dynamic pressure. Results are obtained separately for cases of northward and southward interplanetary magnetic field (IMF) prior to and after the solar wind pressure jump. For pressure enhancements with southward IMF we find that the response of the geosynchronous magnetic field on the dayside is mostly compression. However, on the nightside it can be a dipolarization‐like change that is similar to that which occurs during substorms. We report four events showing the dipolarization‐like response on the nightside. Analysis using low‐latitude geomagnetic data as well as the geosynchronous energetic particle flux data leads us to argue that this dipolarization is part of the global disturbance directly driven by the solar wind pressure enhancement. We also find that the nightside magnetosphere is remarkably sensitive to small solar wind dynamic pressure enhancements when the IMF is strongly southward (i.e., Bz is below ∼−10 nT). For northward IMF, solar wind pressure enhancements generally lead to magnetic compression, with a few cases of depression on the nightside, neither of which is a dipolarization‐like response. The compression is strongest near noon, decreases toward dawn and dusk, and increases with the amount of increase in dynamic pressure.

Modeling the inner plasma sheet protons and magnetic field under enhanced convection

In order to understand the evolution of the protons and magnetic field in the inner plasma sheet from quiet to disturbed conditions, we incorporate a modified version of the Magnetospheric Specification Model (MSM) with a modified version of the Tsyganenko 96 (T96) magnetic field model to simulate the protons and magnetic field under an increasing convection electric field with two‐dimensional (2‐D) force balance maintained along the midnight meridian. The local time dependent proton differential fluxes assigned to the model boundary are a mixture of hot plasma from the mantle and cooler plasma from the low latitude boundary layer (LLBL). We previously used this model to simulate the inner plasma sheet under weak convection corresponding to a cross polar cap potential drop (ΔΦPC) equal to 26 kV and obtained 2‐D quiet time equilibrium for proton and magnetic field that agrees well with observations. We start our simulation for enhanced convection with this quiet time equilibrium and time‐independent boundary particle sources and increase ΔΦPC steadily from 26 to 146 kV in 5 hours. Simulations are also run separately to steady states by keeping ΔΦPC constant after it is increased to 98 and 146 kV. The magnitudes of proton pressure, number density, and temperature and their increase from quiet to moderate activity (ΔΦPC = 98 kV) are consistent with most observations. Our simulation at high activity (ΔΦPC = 146 kV) underestimates the observed pressure and temperature. This disagreement indicates possible dependence of the boundary particle sources on activity and possible effects of solar wind dynamic pressure enhancements that have not yet been included in our simulation. The simulated equatorial pressures and temperatures show stronger enhancement on the dusk side than on the dawn side as convection is increased, while density profiles show an increase on the dawn side and a decrease on the dusk side. The simulated proton flow speed at the equatorial plane increases with enhancing convection while the overall flow direction does not change significantly, a result of enhancement in both the earthward electric drift and the azimuthal diamagnetic drift. The equatorial magnetic field strength decreases more in the near‐Earth plasma sheet than at larger radial distances as ΔΦPC increases, resulting in an increasing flat radial profile with enhancing convection. The feedbacks from diamagnetic drift and magnetic fields to increasing convection are found to restrain the pressure increase. Based on the good agreement between our results and observations at moderate activity, our magnetic field indicates that the plasma and magnetic field in the plasma sheet can be in a state far from possible force balance inconsistency during periods of moderately enhanced convection. A scale analysis of our results shows that the frozen‐in condition E = −v × B is not valid in the inner plasma sheet for moderate activity.

Sudden solar wind dynamic pressure enhancements and dayside detached auroras: IMAGE and DMSP observations

Dayside detached auroras (DDA) refer to auroras observed separate from the equatorward edge of the main oval on the dayside. They are studied here using IMAGE FUV and DMSP particle data. Occurrence of these DDA appears to be correlated with sudden solar wind dynamic pressure enhancements and northward interplanetary magnetic field, as monitored by the Wind satellite. They are usually very dynamic and short‐lived with a lifetime of the order of 10 minutes. Out of the three FUV instrument channels on IMAGE, DDA are best detected by the IMAGE FUV SI‐12 instrument, which measures intensities of the Doppler red‐shifted Hydrogen Lyman Alpha line. This indicates that energetic proton precipitation is the major component. Simultaneous DMSP particle observations confirm that energetic protons (>10 keV) in the dayside inner magnetosphere are the primary source of those DDA detected by the SI‐12 instrument. DMSP also detected significant electron fluxes associated with the DDA, but the electron precipitations have little or no contribution to the DDA intensities detected by the SI‐12 instrument. Precipitations of energetic protons (electrons) which caused DDA could be explained by enhanced cyclotron instability which arose from adiabatic compression following sudden solar wind dynamic pressure enhancements.

Auroral disturbances during the January 10, 1997 magnetic storm

It is well known that intense and frequent auroral‐zone disturbances, often attributed to substorms, occur during magnetic storms. We examine observations during the January 10, 1997 main phase and find that observed auroral‐zone activity was dominated by a combination of global auroral and current enhancements, which are a direct response to solar wind dynamic pressure enhancements, and poleward boundary intensifications, which are localized in longitude and have an auroral signature that moves equatorward from the magnetic separatrix. Poleward and azimuthally expanding regions of auroral activity which accompany substorms are found to contribute significantly less to the observed activity. This suggests that poleward boundary intensifications and dynamic pressure responses may be an important cause of disturbances during periods of enhanced convection such as magnetic storms and convection bays.

Geomagnetic disturbances: characteristics of, distinction between types, and relations to interplanetary conditions

This tutorial emphasis disturbances in auroral emissions and ionospheric currents and their relation to interplanetary conditions and the overall level of geomagnetic activity. Auroral zone disturbances are divided into three fundamentally different types: poleward boundary intensifications (PBIs), substorms, and effects of solar wind dynamic pressure enhancements. The most common type of auroral-zone disturbance is the PBI, which occurs during all levels of geomagnetic activity. PBIs have an auroral signature that often can be seen to move equatorward from the magnetic separatrix. They are typically associated with ground magnetic perturbations of few tens of nT, but perturbations can be as high as ∼500 nT . Individual PBIs are longitudinally localized, associated with the longitudinally localized flow bursts in the tail plasma sheet, and occasionally traverse essentially the entire latitudinal extent of the plasma sheet. PBIs appear to generally be the dominant type of auroral-zone disturbance during periods of enhanced magnetospheric convection, including the growth phase of substorms, convection bays, and the main phase of magnetic storms. Substorms are a far more dramatic and large scale, but far less common, disturbance than PBIs. They occur after a ≳30 min growth-phase period of enhanced convection. It is now known that at least ∼50% of substorms are associated with IMF changes that lead to a reduction in the strength of convection. However, it has not yet been shown whether or not all are most substorm onsets are caused by these types of IMF changes. Auroral activity during substorms typically initiates within a ∼1–2 h MLT sector near the equatorward boundary of the auroral oval and then expands both poleward and azimuthally. Substorms are associated with ground magnetic disturbances that range from ∼50 to ∼2000 nT , a reduction in strength of the cross-tail current, a poleward displacement of the inner edge of the plasma sheet, and a large release of plasma and magnetic field energy from the region earthward of the new inner edge of the plasma sheet. The reduction of cross-tail current is also believed to often be associated with a severance, and loss from the magnetotail, of the outer portion of the plasma sheet ( r≳25 R E). Recent studies have shown that solar wind dynamic pressure increases caused large auroral-zone disturbances during a stormtime period of strongly enhanced convection, affecting the poleward boundary, latitudinal width, and intensity of the auroral oval. Dynamic pressure increases also appear to also enhance the entire magnetospheric current system, including the magnetopause, cross-tail, region 1 field-aligned, and global ionospheric currents. Thus, in addition to PBIs, significant variations in solar wind dynamic pressure should be considered as a possibly important source of geomagnetic disturbances during periods of enhanced magnetospheric convection.

Magnetopause location under extreme solar wind conditions

During the solar wind dynamic pressure enhancement, around 0200 UT on January 11, 1997, at the end of the January 6–11 magnetic cloud event, the magnetopause was pushed inside geosynchronous orbit. The LANL 1994–084 and GMS 4 geosynchronous satellites crossed the magnetopause and moved into the magnetosheath. Also, the Geotail satellite was in the magnetosheath while the Interball 1 satellite observed magnetopause crossings. This event provides an excellent opportunity to test and validate the prediction capabilities and accuracy of existing models of the magnetopause location for producing space weather forecasts. In this paper, we compare predictions of two models: the Petrinec and Russell [1996] model and the Shue et al. [1997] model. These two models correctly predict the magnetopause crossings on the dayside; however, there are some differences in the predictions along the flank. The Shue et al. [1997] model correctly predicts the Geotail magnetopause crossings and partially predicts the Interball 1 crossings. The Petrinec and Russell [1996] model correctly predicts the Interball 1 crossings and is partially consistent with the Geotail observations. We further found that some of the inaccuracy in Shue et al.'s predictions is due to the inappropriate linear extrapolation from the parameter range for average solar wind conditions to that for extreme conditions. To improve predictions under extreme solar wind conditions, we introduce a nonlinear dependence of the parameters on the solar wind conditions to represent the saturation effects of the solar wind dynamic pressure on the flaring of the magnetopause and saturation effects of the interplanetary magnetic field Bz on the subsolar standoff distance. These changes lead to a better agreement with the Interball 1 observations for this event.

Dayside auroral activities and their implications for impulsive entry processes in the dayside magnetosphere

Dayside aurorae often exhibit transient features with latitudinal widths of about 100–200 km, longitudinal extents of < 500 km, and durations of about 1–10 min. Many researchers regard such auroral activity as evidence for direct, impulsive entry of magnetosheath plasma into the magnetosphere via magnetopause penetration across closed field lines or flux transfer events (FTEs) on open field lines. Recent work suggests that a number of previously reported low-altitude transient events in the auroral zone could be directly associated with solar wind dynamic pressure variations (either increases or decreases) in which the dynamic pressure is seen to change by up to a factor of 3 for a duration of 1–10 min. In this paper, we first investigate the dayside auroral bright spots as seen by the Viking satellite and discuss evidence which argues against the notion that these auroral bright spots are the optical signatures of FTEs in the ionosphere. We then re-examine past reports of transient dayside auroral phenomena which were interpreted as the optical signatures of FTEs to determine the solar wind conditions at the time of the events. We find that many of the previously reported transient occurrences of poleward drifting auroral arcs in the dayside auroral region can be associated with sudden increases in the solar wind dynamic pressure. An explanation of these events in terms of solar wind dynamic pressure enhancement is proposed

Extreme ultraviolet emissions for monitoring auroras in dark and daylight hemispheres

Observations of auroral extreme ultraviolet (EUV) emissions from the noon‐midnight polar‐orbiting Space Test Program (STP) 78‐1 satellite are used to investigate the spectral features of the dayside auroral oval, nightside auroral oval, and the dayglow. The purpose of this study is to evaluate the possibility of using EUV atmospheric emissions for global auroral imaging and to study the temporal variations of the auroral EUV intensity with geomagnetic activity. It is found that during geomagnetically quiet times, O II 834Å, O I 989Å, and N2 1009 Å measurements can provide a clear distinction of the auroral oval above the dayglow background. During the very active period, five other EUV emissions can give good aurora‐to‐dayglow ratios. The auroral EUV emissions are more intense during magnetospheric substorms; however, we have not found a clear one‐to‐one relationship between them. It is also found that the auroral oval brightness, especially the noonside, increased during the initial phase of a geomagnetic storm. Thus, the sudden solar wind dynamic pressure enhancement is another important geophysical parameter that affects the EUV auroral emission.