Dynamic Pressure Enhancements Research Articles

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.

Solar wind dynamic pressure enhancements and polar auroras

Polar auroras are observed to follow sudden enhancements in solar wind dynamic pressure. In auroral studies conducted at Eureka NWT (89° magnetic latitude), the time delay from the pressure enhancement to appearance of an auroral arc was found to depend on the sign of the interplanetary magnetic field (IMF) B x component. For 15 cases with B x northward the average time delay was 70±28 min consistent with a closed magnetosphere and the pressure enhancement causing a compression in the magnetotail. For the remaining 10 cases with B x southward the average time delay was 18±10 min consistent with reconnection between the IMF and the magnetosphere and direct penetration of particles into the polar cap. The pressure enhancements usually occurred with strong IMF and transient changes in B y . This suggests that transient changes in B y may be a factor in the auroral triggering.

Cusp/cleft auroral activity in relation to solar wind dynamic pressure, interplanetary magnetic field Bz and By

Continuous optical observations of cusp/cleft auroral activities within ≈ 09‐15 MLT and 70‐76° magnetic latitude are studied in relation to changes in solar wind dynamic pressure and interplanetary magnetic field (IMF) variability. The observed latitudinal movements of the cusp/cleft aurora in response to IMF Bz changes may be explained as an effect of a variable magnetic field intensity in the outer dayside magnetosphere associated with the changing intensity of region 1 field‐aligned currents and associated closure currents. Ground magnetic signatures related to such currents were observed in the present case (January 10, 1993). Strong, isolated enhancements in solar wind dynamic pressure (Δp/p ≥ 0.5) gave rise to equatorward shifts of the cusp/cleft aurora, characteristic auroral transients, and distinct ground magnetic signatures of enhanced convection at cleft latitudes. A sequence of auroral events of ≈ 5‐10 min recurrence time, moving eastward along the poleward boundary of the persistent cusp/cleft aurora in the ≈ 10‐14 MLT sector, during negative IMF Bz and By, conditions, were found to be correlated with brief pulses in solar wind dynamic pressure (0.1 < Δp/p < 0.5). Simultaneous photometer observations from Ny Ålesund, Svalbard, and Danmarkshavn, Greenland, show that the events often appeared on the prenoon side (≈ 10‐12 MLT), before moving into the postnoon sector in the case we study here, when IMF By < 0. In other cases, similar auroral event sequences have been observed to move westward in the prenoon sector, during intervals of positive By. Thus a strong prenoon/postnoon asymmetry of event occurence and motion pattern related to the IMF By polarity is observed. We find that this category of auroral event sequence is stimulated bursts of electron precipitation that originate from magnetosheath plasma that has accessed the dayside magnetosphere in the noon or near‐noon sector, possibly at high latitudes, partly governed by the IMF orientation as well as by solar wind dynamic pressure pulses.

Motion of the heliospheric termination shock: 3. Incident interplanetary shocks

In this paper the response of the heliospheric termination shock to an incident interplanetary shock is examined. This paper is an extension of a recent study by Barnes (1993), which treated the analogous problem for an incident contact discontinuity. The termination shock is treated as a strong gasdynamic shock. The postinteraction configuration consists of a moving termination shock, a postshock contact discontinuity, and either a shock or rarefaction wave propagating the disturbance signal into the downstream medium. For a decrease in dynamic pressure a rarefaction wave propagates downstream, and the new termination shock propagates inward, while for an enhancement of dynamic pressure the termination shock moves outwards and a weak outer shock propagates into the downstream medium; speeds of motion of the termination shock are typically of the order of ∼100 km/s. The results are similar to those presented by Barnes (1993) indicating that the results of that paper are robust within the gasdynamic model, in the sense of being independent of the details of the initial disturbance.

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.