Research On The Evolution Of Mercury's Substorms Based On MESSENGER Observations

Magnetospheric substorm,as a basic mode of magnetospheric perturbation,is regarded as an ideal model for understanding the energy release process of plasma in the universe.Mercury is the smallest planet in the solar system and the closest one to the sun.Therefore,study of the evolution process of Mercury's magnetospheric substorm is helpful for understanding the energy release process in the magnetosphere of different planets.In this paper,four years of on-orbit observations of MESSENGER were used to study the substorm-related phenomena occurring in Mercury's magnetosphere.In this paper,100 substorm events occurring in the Mercury`s magnetospheric plasma sheet are identified.Using this database,we first studied the duration,magnetic flux variations and spatial distribution of the substorm growth and expansion phase statistically.The results are: the average duration of Mercury's substorm growth phase and expansion phase is 82.68 s and 94.36 s,respectively,which is only 1/100 ? 1/50 of the corresponding phase duration of Earth's magnetospheric substorm.the open magnetic flux value before the energy unloading of Mercury's substorm is very large,which indicates that there is no open magnetic flux threshold for energy unloading during Mercury's substorm process.This will lead to the rapid energy loading and unloading processes during Mercury's magnetospheric substorms.The average magnetic flux released during Mercury's substorms accounts for about 11% of Mercury's total magnetic flux,which is much higher than that during Earth's substorm,which is 2.6%.Mercury's substorms were more often observed in the southern hemisphere and on the dusk side of the plasma sheet.Combining the distribution of Mercury's substorm with the duration and magnetic flux variation,we found that,on the dusk side of the plasma sheet,both the average duration and magnetic flux release of Mercury's substorm are larger than those on the dawn side.However,the magnetic flux release rates are comparable.We suggest that the mechanisms of Mercury's magnetospheric substorms occurring on both sides are similar.The differences of average duration and magnetic flux release are caused by the different magnetic activities and magnetic pressures on the two sides.We observed multiple energy release processes during the expansion phase of Mercury's magnetospheric substorm as well as continuous dipolarization fronts in the plasma sheet for the first time.On September 30,2014,four successive global magnetic dipolarizations are observed,accompanying by the magnetic energy release process.We propose that the possible reason is that the Bursty Bulk Flow(BBF)generated by multiple magnetic reconnection processes occurring at Mercury's far magnetotail transported a large amount of plasma and magenetic flux to the near-Mercury magnetotail,causing the multiple global dipolarizations in the near-Mercury's magnetosphere.The continuous dipolarization fronts in the magnetospheric plasma sheet is also caused by the multiple reconnections occurring at mid and far magnetotail.However,the reconnection generated BBFs do not carry enough magnetic flux to drive the global magnetospheric substorms as magnetic reconnection process is much weaker.The analysis results of this paper are helpful for further understanding the characteristics of the evolution process of Mercury's magnetospheric substorms and lay a foundation for more comparative studies on planetary magnetospheric physics in the future.