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摘要
During geomagnetic storms, the coupling magnetosphere-ionosphere-thermosphere system is a rather complex phenomenon, and the thermospheric mass density exhibits large deviations from the climatological behavior upon the conjunct effect of Joule/particle heating, Lorentz force, thermal expansion, upwelling, and horizontal wind circulation. Due to different weight effects, thermospheric responses might vary with different storms, and even for the same storm case resulting from unlike methods of data process. In order to know more about the seasonal, magnetic local time (MLT) and latitude dependencies and the time delay characteristic of the thermospheric response to geomagnetic storms, we investigate the thermospheric response to 267 geomagnetic storms in which the Dst minimum, Dst(min), is below -50 nT during 2002-2008. The data of thermospheric mass density normalized to 400 km is derived from the high-accuracy accelerometer on board the CHAMP satellite. Each orbit is first divided into an ascending and a descending half, which are subdivided into five latitudinal segments, namely +/- 60 degrees, +/- 30 degrees and 0 degrees. In order to investigate the dependence of MLT, density data are sorted into 4 different MLT sectors: 05 00MLT to 09 00MLT as the dawn sector, 10 00MLT to 16 00MLT as the noon sector, 17 00MLT to 21 00MLT as the dusk sector, and 22 :00MLT to 04 00MLT as the night sector. To investigate seasonal variations, the available data are subdivided into three local seasons: the northern hemisphere winter (December-February, DJF), combined equinoxes (March-May, MAM, and September-November, SON), and the northern hemisphere summer (June-August, JJA). Dst(min) is used to identify four categories of geomagnetic storms: weak storms (-30>Dsttmin >=-50 nT), moderate storms (-50>Dst(min >=)-100 nT), intense storms (-100>Dst(min)>=-200 nT) and great storms (Dst(min)<-200 nT). By this means the effects of magnetic local time, latitude, season and intensity of storm are separated. Since the quiet-time density (rho(q)) shows much dependence on the solar activity, season, and local time, the density deviation from quiet-time values, rather than the total storm-time density (rho) itself, seems better suited for describing the storm effect. There are two ways to define the deviation, one is the absolute difference (delta rho(a) = rho - rho(q)), and the other is the percentage difference (delta rho(r) = delta rho(a)/rho(d)). As there is no general agreement on which expression is more appropriate, both the absolute and the percentage variations for each event are presented to produce a complete picture. Considering that the MSIS model underestimates the total mass density in the crest region resulting from its missing double peaks at low latitudes completely, the CHAMP measurements from the day prior to the storm is taken as a quiet-time reference density. The thermospheric mass density reacts after geomagnetic activity with a delay time, which is expected to depend on latitudes, MLT and seasons. Besides the superposed epoch comparisons for different conditions during storms, in which epoch time zero is chosen as the time of Dst(min) time delays between Dst(min) and maxima of densities which are divided into different season, latitude, and MLT, have been computed for each storm event and the statistical result accounted for the biggest proportion describes quantitatively the time intervals. Besides some characteristics that have been mentioned in previous research, our statistical results reveal some new or more detailed variations about the responses of thermospheric mass density to geomagnetic storms, and the main conclusions are as follows: 1) The absolute enhancements of thermospheric density during storms show a north-south asymmetry dependence on both. the intensity of storms and the magnetic local time. In the northern hemisphere summer, for great storms and the nightside of moderate storms, controlled by higher Joule heating rates and prevailing summer-to-winter winds, stronger density enhancements occur in the summer hemisphere. On the dayside of northern hemisphere summer, due to the faster propagation of the disturbance from high to low latitudes in the summer hemisphere, the thermospheric density enhancements happen near 30 degree in the northern (summer) hemisphere peak ahead of those in the southern (winter) hemisphere. While probably affected by the higher rate of the energy transferred to the thermosphere partly dependent on the strength of the background magnetic field which is weaker in the Southern hemisphere due to shifted position of the dipole in positive Z-direction, on the dayside of northern hemisphere summer during intense and moderate storms, delta rho a of the southern ( winter) hemisphere was stronger than that of the northern (summer) hemisphere, and on the dayside near equinoxes for most storms, the thermospheric density enhancements near 30 degree of the northern hemisphere peaked 1 similar to 2 h ahead of that of the southern hemisphere. 2) Thermospheric densities of low latitudes enhancing after that of high latitudes during storms, the delay time during great storms is shorter than that of other weaker storms, and the time-lag during nightsie is shorter than that of dayside, indicating that propagation of energy deposited in polar regions to lower latitudes seems faster in the night-side sector during stronger storms. Only for great storms, the percentage difference delta rho r of dayside sector in low latitudes is higher than that of high latitudes, and the density of low latitudes peaks earlier than that of high latitudes, implying some other heating source in low latitudes play an important role during great storms. 3) Affected by the storm-time disturbance-driven thermospheric meridional circulation, the thermospheric density enhancements of the equator approach their maxima fastest at the equinoxes, and the time delay relative to Dst(min) is 1 h, 2 h for the density of dayside, night-side, respectively. At the nightside either in summer or in winter, the thermospheric density of the equator tends to peak 3 h after Dst(min). While for the dayside, the time interval that thermospheric density at the equator approached its maximum is dependent on seasons, and it is shortest for the northern hemisphere winter. 4) At dayside, the thermospheric density enhancement at the equator tends to peak after 3h the density of 60oapproached its maximum, which is independent of seasons. While at nightside of equinoxes and northern hemisphere winter, the thermospheric density at the equator tends to peak before that of high latitudes done, meanwhile the density enhancement maxima of those latitudes were comparable, implying some other heating source working. Although the thermospheric density at the equator tends to respond with 0 similar to 2 h delay relative to the response of Dst index during most storms, while in some cases, the density at the equator enhances before the Dst index responded.
- 出版日期2015-9
- 单位中国科学院地质与地球物理研究所; 中国科学院地质与地球物理研究所兰州油气资源研究中心
