J. K. Chao

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 tinder 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 B z on the subsolar standoff distance. These changes lead to a better agreement with the Interball 1 observations for this event.

A new functional form to study the solar wind control of the magnetopause size and shape

In this study a new functional form, r = r 0 [2/(1 + cos θ)] α , is used to fit. the size and shape of the magnetopause using crossings from ISEE 1 and 2, Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM), and IMP 8 satellites. This functional form has two parameters, τ 0 and α, representing the standoff distance and the level of tail flaring. The value r is the radial distance at an angle (θ) between the Sun-Earth line and the direction of τ. It is found that r 0 varies with the interplanetary magnetic field (IMF) B z component and has a break in the slope at B z = 0 nT. The best-fit value of τ 0 decreases with increasing southward IMF B z . For northward IMF B z , the best-fit value of τ 0 increases slightly with increasing B z . The best-fit value of α increases monotonically with decreasing IMF B z . The dynamic pressure (D p ) also changes τ 0 and α. The parameters D p and τ 0 are related by a power law of -1/(6.6±0.8). The best-fit value of α is slightly larger for larger dynamic pressure, which implies that D p also has a role in flux transfer from the dayside to the nightside, but the size of this effect is small. An explicit function for the size and shape of the magnetopause, in terms of D p and B z , is obtained by using multiple parameter fitting in a form that is useful for operational space applications such as predicting when satellites at geosynchronous orbit will be found in the magnetosheath.

Influence of the solar wind dynamic pressure on the decay and injection of the ring current

The influence of the solar wind dynamic pressure on the decay and injection of thering current is investigated empirically, on the basis of the solar wind and the geomagneticindex Dst of the OMNI database, for the period from January 1964 to July 2001. Wefound that when the position of the ring current is closer to the Earth for a higher solarwind dynamic pressure, the decay time of the ring current decreases. The decay time, inhours, varies as follows, t = 8.70 exp(6.66/(6.04 + P)), for northward interplanetarymagnetic fields (IMF), where P is the solar wind dynamic pressure in nanopascals. It isalso found, by minimizing the root mean square errors of the hourly Dst differencebetween the calculated values and the measured ones, that the ring current injection rate isproportional to the solar wind dynamic pressure, with a power index equal to 0.2 duringsouthward IMF. This implies that the ring current injection increases when themagnetosphere is more compressed by high solar wind dynamic pressure. On the basis ofour new results we demonstrate that the predictions of Dst using O’Brien andMcPherron’s [2000a] model are improved, especially for intense geomagnetic stormswhen the influence of the solar wind dynamic pressure on the decay and injection of ringcurrent is taken into consideration.

Models for the size and shape of the earth's magnetopause and bow shock

New models for the size and shape of the Earths magnetopause and bow shock are derived, based on a criterion for selecting the crossing events and their corresponding up-stream solar wind parameters. In this study, we emphasize the importance of accurate interplanetary parameters for, predicting the size and shape of the magnetopause and bow shock. The time lag of the solar wind between the solar wind monitor and the location ofcrossings is carefully considered, ensuring more reliable up-stream solar wind parameters. With this database new functional forms for the magnetopause and bow shock surfaces are deduced. In this paper, we briefly present the preliminary results. For a given up-stream solar wind dynamic pressure Dp, an IMF norths-south compoent Bz, a solar wind β and a magnetosconic mach number Mms, the parameters that describe the magnetopause and bow shock surfaces ro and α can be expressed in terms of a set of coefficients determined with a multi-parameter fitting. Applications of these models to extreme solar wind conditions are demonstrated. For convenience, we have assumed that ro, Bz and Dp retain their units, except in equations where they are normalized by 1 RE(Earth radius), 1 nT and 1 nPa, respectively.

Magnetospheric response to solar wind dynamic pressure variations: Interaction of interplanetary tangential discontinuities with the bow shock

Some magnetic impulse events observed in the polar region are related to vortices associated with plasma convection in the ionosphere. Recent analyses of satellite and ground data suggest that the interaction of solar wind dynamic pressure pulses and the magnetosphere may lead to the formation of velocity vortices in the magnetopause boundary layer region. This can in turn lead to the presence of vortices in the polar ionosphere. However, before reaching the Earths magnetopause, these interplanetary pressure pulses must interact with and pass through the bow shock. A variation of the solar wind dynamic pressure (ΔρV²) may be associated with shocks, magnetic holes, or tangential discontinuities (TDs) in the interplanetary medium. We study the interaction of interplanetary TDs with the Earths bow shock (BS) using both theoretical analysis and MHD computer simulations. It is found that as a result of the collision between a TD and the BS, the jump in the solar wind dynamic pressure associated with the TD is significantly modified, the bow shock moves, and a new fast shock or fast rarefaction wave, which propagates in the downstream direction, is excited. Our theoretical analysis shows that the change in the plasma density across the interplanetary TD plays the most important role in the collision process. In the case with an enhanced dynamic pressure behind the interplanetary TD, the bow shock is intensified in strength and moves in the earthward direction. The dynamic pressure jump associated with the transmitted TD is generally reduced from the value before the interaction. A fast compressional shock is excited ahead of the transmitted TD and propagates toward the Earths magnetosphere. For the case in which the dynamic pressure is reduced behind the interplanetary TD, the pressure jump across the transmitted TD is substantially weakened, the bow shock moves in the sunward direction, and a rarefaction wave which propagates downstream is excited. We also simulate and discuss the interaction of a pair of tangential discontinuities, which may correspond to a magnetic hole, with the BS.

Size and energy distributions of interplanetary magnetic flux ropes

In observations from 1995 to 2001 from the Wind spacecraft, 144 interplanetary magnetic flux ropes were identified in the solar wind around 1 AU. Their durations vary from tens of minutes to tens of hours. These magnetic flux ropes include many small- and intermediate-sized structures and display a continuous distribution in size. Energies of these flux ropes are estimated and it is found that the distribution of their energies is a good power law spectrum with an index similar to - 0.87. The possible relationship between them and solar eruptions is discussed. It is suggested that like interplanetary magnetic clouds are interplanetary coronal mass ejections, the small- and intermediate-sized interplanetary magnetic flux ropes are the interplanetary manifestations of small coronal mass ejections produced in small solar eruptions. However, these small coronal mass ejections are too weak to appear clearly in the coronagraph observations as an ordinary coronal mass ejection.

Comparative study of bow shock models using Wind and Geotail observations

Wind and Geotail observed bow shock (BS) crossings were selected from the 1998 to 2001 ISTP database. We analyzed 625 case events containing 4381 Geotail BS crossings and 130 case events containing 917 Wind BS crossings. The location of the BS crossings, in the aberrated GSE coordinate system, varied over a wide range from -85 Re to 45 Re along the X-GSE axis, up to 90 Re in the perpendicular direction. ACE, Wind, and Geotail measurements were used to determine the upstream solar wind conditions in the interplanetary medium. These conditions were determined for the BS crossings in each case event by using the delay time of direct solar wind propagation from an upstream monitor to the probe satellite (Wind or Geotail). The solar wind conditions for the BS crossings varied over a wide range of dynamic pressures Pd (from 0.02 nPa to 49 nPa), IMF Bzs (from -26 nT to 23 nT), thermal/magnetic pressure ratios beta (from 0.002 to 50), and magnetosonic Mach numbers M(ms) (from 1.02 to 29). Such a wide spatial and dynamic range of BS crossings permits us to consider the different parameters that control the BS size and shape, such as the radius of curvature of the magnetopause which depends on Pd and Bz, the Alfven, sonic, and magnetosonic Mach numbers, and the IMF orientation. To study the dependence on these parameters, we compared the accuracy of the BS models formulated by Peredo et al. [1995], Russell and Petrinec [1996], Verigin et al. [2001b], and Chao et al. [2002] for the prediction of selected BS crossings observed in different bow shock regions and with various upstream solar wind conditions. It was found that the Chao et al. [2002] model had the best capability for predicting the BS crossings. The solar wind dynamic pressure and magnetosonic Mach number were determined to be the most important parameters controlling the BS size and shape. The important role of the dawn-dusk asymmetry of the bow shock tail region is emphasized. The effect of the southward IMF influence on the dayside magnetosheath thickness is revealed and discussed.

Interplanetary small‐ and intermediate‐sized magnetic flux ropes during 1995–2005

We present a comprehensive survey of 125 small- and intermediate-sized interplanetary magnetic flux ropes during solar cycle 23 (1995-2005) using Wind in situ observations near 1 AU. As a result, we found the following: (1) The annual number of small- and intermediate-sized interplanetary magnetic flux ropes is not very sensitive to the solar cycle, but its trend is very similar to that of magnetic clouds (MCs). (2) Average speeds of the individual small- and intermediate-sized interplanetary magnetic flux ropes varied from 289 to 790 km/s with a mean value of 420 +/- 86 km/s. Most small- and intermediate-sized interplanetary magnetic flux ropes were found to have a propagation speed similar to typical slow speed solar wind speed, and only a few small- and intermediate-sized interplanetary magnetic flux ropes had speeds comparable to the typically high speed solar wind. (3) Average magnetic field strength for small- and intermediate-sized interplanetary magnetic flux ropes is less than the average magnetic field strengths of MCs, while it is larger than that of background solar wind. (4) The distributions of the axial orientations for small- and intermediate-sized interplanetary magnetic flux ropes are also similar to that of MCs. The results show that small- and intermediate-sized interplanetary magnetic flux ropes and MCs have many similar (or relative) characters. So we suggest that both MCs and small- and intermediate-sized interplanetary magnetic flux ropes originate from solar eruptions.

Toward predicting the position of the magnetopause within geosynchronous orbit

Although the average magnetopause is ∼10 R E from the Earth, the magnetopause moves inside the geosynchronous orbit during extreme solar wind conditions. Under these circumstances some geosynchronous satellites suddenly enter the magnetosheath and are exposed to the plasma and fields of the magnetosheath. In this study we evaluate the predictive capabilities of magnetopause location models in forecasting geosynchronous magnetopause crossings. We predict periods during which geosynchronous satellites enter the magnetosheath using the Petrinec and Russell [1996] and Shue et al. [1998] magnetopause location models driven by data from Interplanetary Monitor Platform (IMP) 8. These predictions are then verified with in situ observations from Geosynchronous Operational Environment Satellite (GOES) 5, 6, and 7. We estimate the false alarm rate, probability of detection, and probability of false prediction for the two models. The estimation shows that false alarm rate for a forecast with a 20-min separation cadence is ∼62% (80%) for the Shue et al. [1998] model (the Petrinec and Russell [1996] model). The probability of detection is very high for both prediction models. These results suggest that both models work well in predicting magnetosheath periods for geosynchronous satellites. Predictions from the models provide a prerequisite condition for geosynchronous magnetopause crossings. Further examination of unsuccessful events indicates that preconditioning by the interplanetary magnetic field B z needs to be included in the forecasting procedure for a better forecast. This finding provides us with a guide to improving future magnetopause location models.

Model of auroral electron acceleration by dissipative nonlinear inertial Alfven wave

In a recent work [Wu, 2003a, 2003b], a dissipative nonlinear inertial Alfven wave (DNIAW) were proposed as the physical explanation for the formation of the strong electric spikes often observed in the auroral ionosphere and the magnetosphere. DNIAW can also lead to the field_aligned electron acceleration. In the present paper, dynamical characteristics of DNIAW acceleration are discussed and its possible role in auroral electron acceleration is further investigated. The effective acceleration region for auroral electrons with energies of the order of keV produced by DNIAW acceleration is between 0.5 and 2.5 R-E above the ionosphere, and the most efficient acceleration occurs around 0.8 R-E where both the Alfven velocity and the produced auroral electron energy peak, and the peak energy is around 10 keV. We suggest that this could explain the precipitous decrease of the auroral electron energy spectrum toward energies above 10 keV, which can be inferred from measurements of energy distribution of precipitating auroral electrons. Typical widths of auroral arcs caused by the DNIAW acceleration are in scales of the order of 1 km.