Assessing the performance of magnetopause models based on THEMIS data

The magnetopause is the boundary between the Earth’s magnetic field and the interplanetary magnetic field(IMF),located where the supersonic solar wind and magnetospheric pressure are in balance.Although empirical models and global magnetohydrodynamic simulations have been used to define the magnetopause,each of these has limitations.In this work,we use 15 years of magnetopause crossing data from the THEMIS(Time History of Events and Macroscale Interactions during Substorms)spacecraft and their corresponding solar wind parameters to investigate under which solar wind conditions these models predict more accurately.We analyze the pattern of large errors in the extensively used magnetopause model and show the specific solar wind parameters,such as components of the IMF,density,velocity,temperature,and others that produce these errors.It is shown that(1)the model error increases notably with increasing solar wind velocity,decreasing proton density,and increasing temperature;(2)when the cone angle becomes smaller or|Bx|is larger,the Shue98 model errors increase,which might be caused by the magnetic reconnection on the dayside magnetopause;(3)when|By|is large,the error of the model is large,which may be caused by the east-west asymmetry of the magnetopause due to magnetic reconnection;(4)when Bz is southward,the error of the model is larger;and(5)the error is larger for positive dipole tilt than for negative dipole tilt and increases with an increasing dipole tilt angle.However,the global simulation model by Liu ZQ et al.(2015)shows a substantial improvement in prediction accuracy when IMF Bx,By,or the dipole tilt cannot be ignored.This result can help us choose a more accurate model for forecasting the magnetopause under different solar wind conditions.

The dawn−dusk asymmetry in mesosphere and lower thermosphere temperature disturbances during geomagnetic storms at high latitude

Utilizing observations by the Sounding of the Atmosphere using Broadband Emission Radiometry(SABER)instrument,we quantitatively assessed the dawn-dusk asymmetry in temperature disturbances within the high-latitude mesosphere and lower thermosphere(MLT)during the main phase of geomagnetic storms in this study.An analysis of five geomagnetic superstorm events indicated that during the main phase,negative temperature disturbances were more prevalent on the dawn side than on the dusk side in the high-latitude MLT region.Results of a statistical analysis of 54 geomagnetic storm events also revealed a notable disparity in temperature disturbances between the dawn and dusk sides.At high latitudes,38.2%of the observational points on the dawn side exhibited negative temperature disturbances(less than−5 K),whereas on the dusk side,this percentage was only 29.5%.In contrast,at mid-latitudes,these proportions were 34.1%and 36.5%,respectively,showing no significant difference.We also conducted a statistical analysis of temperature disturbances at different altitudes,which revealed an increase in the proportion of warming disturbances with altitude.Conversely,the proportion of cooling disturbances initially rose with altitude,reaching a peak around 105 km,and subsequently decreased.These temperature disturbance differences could be explained by the day-night asymmetry in vertical wind disturbances during storm conditions.

Magnetosphere response to the IMF turning from north to south

In this paper, the Space Weather Modeling Framework(SWMF) is used to simulate the real-time response of the magnetosphere to a solar wind event on June 5, 1998, in which the interplanetary magnetic field shifted its direction from north to south.Since most current models do not take into account convective effects of the inner magnetosphere, we first study the importance of Rice Convection Model(RCM) in the global model.We then focus on the following four aspects of the magnetosphere's response: the magnetosphere's density distribution, the structure of its magnetic field lines, the area of the polar cap boundary, and the corresponding ionospheric current change.We find that(1) when the IMF changes from north to south in this event, high magnetosheath density is observed to flow downstream along the magnetopause with the solar wind; low-latitude reconnection at dayside occurs under the southward IMF, while the magnetic field lines in the tail lobe caudal, caused by the nightside high latitude reconnection, extend into the interplanetary space.Open magnetic field lines exist simultaneously at both high and low latitudes at the magnetopause;(2) the area of the polar cap is obviously increased if the IMF turns from the north to the south; this observation is highly consistent with empirical observations;(3) the ionospheric field align current in the northern hemisphere is stronger than in the southern hemisphere and also increases as the IMF changes from north to south.SWMF with the Rice Convection effect provides reliable modeling of the magnetospheric and ionospheric response to this solar wind variation.

Formation of the bow shock indentation: MHD simulation results

Simulation results from a global magnetohydrodynamic(MHD)model are used to examine whether the bow shock has an indentation and characterize its formation conditions,as well as its physical mechanism.The bow shock is identified by an increase in plasma density of the solar wind,and the indentation of the bow shock is determined by the shock flaring angle.It is shown that when the interplanetary magnetic field(IMF)is southward and the Alfvén Mach number(Mα)of solar wind is high(>5),the bow shock indentation can be clearly determined.The reason is that the outflow region of magnetic reconnection(MR)that occurs in the low latitude area under southward IMF blocks the original flow in the magnetosheath around the magnetopause,forming a high-speed zone and a low-speed zone that are upstream and downstream of each other.This structure hinders the surrounding flow in the magnetosheath,and the bow shock behind the structure widens and forms an indentation.When Mαis low,the magnetosheath is thicker and the disturbing effect of the MR outflow region is less obvious.Under northward IMF,MR occurs at high latitudes,and the outflow region formed by reconnection does not block the flow inside the magnetosheath,thus the indentation is harder to form.The study of the conditions and formation process of the bow shock indentation will help to improve the accuracy of bow shock models.

Statistical study of “trunk-like” heavy ion structures in the inner magnetosphere

We present a statistical study of“trunk-like”structures observed in He+and O+in the inner magnetosphere.The main characteristic of these structures is that the energy of the peak flux decreases earthward.Using observations from the Helium Oxygen Proton Electron(HOPE)instrument onboard Van Allen Probe A,we identify the trunks observed from November 2012 to June 2019 and obtain the universal time,L shell,magnetic local time(MLT),and energy information of each trunk’s root and tip.We then investigate the behavior of trunks in terms of their frequency of occurrence,temporal evolution,spatial and energy distribution,and trunk dependence on different geomagnetic indices.We find that(1)the trunks are always located at L=1.5−4.0 and have a preferential location mainly concentrated at MLT=18−24,(2)the sector within MLT=14−16 is a forbidden zone without trunk roots,and(3)the energy of He+ trunks is the largest near dusk and gradually decreases in the counterclockwise direction,whereas the energy of O+ trunks is relatively evenly distributed with MLT and L.The differences between He+ and O+ trunks are probably due to the different charge exchange and Coulomb collision lifetime.The dependence on different geomagnetic indices indicates that the trunk structures occur more frequently during relatively quiet periods.