Jianpeng Guo

Energy transfer during intense geomagnetic storms driven by interplanetary coronal mass ejections and their sheath regions

The interaction of the solar wind and Earths magnetosphere is complex, and the phenomenology of the interaction is very different for interplanetary coronal mass ejections (ICMEs) compared to their sheath regions. In this paper, a total of 71 intense (Dst <= -100 nT) geomagnetic storm events in 1996-2006, of which 51 are driven by ICMEs and 20 by sheath regions, are examined to demonstrate similarities and differences in the energy transfer. Using superposed epoch analysis, the evolution of solar wind energy input and dissipation is investigated. The solar wind-magnetosphere coupling functions and geomagnetic indices show a more gradual increase and recovery during the ICME-driven storms than they do during the sheath-driven storms. However, the sheath-driven storms have larger peak values. In general, solar wind energy input (the epsilon parameter) and dissipation show similar trends as the coupling functions. The trends of ion precipitation and the ratio of ion precipitation to the total (ion and electron) are quite different for both classes of events. There are more precipitating ions during the peak of sheath-driven storms. However, a quantitative assessment of the relative importance of the different energy dissipation branches shows that the means of input energy and auroral precipitation are significantly different for both classes of events, whereas Joule heating, ring current, and total output energy display no distinguishable differences. The means of electron precipitation are significantly different for both classes of events. However, ion precipitation exhibits no distinguishable differences. The energy efficiency bears no distinguishable difference between these two classes of events. Ionospheric processes account for the vast majority of the energy, with the ring current only being 12%-14% of the total. Moreover, the energy partitioning for both classes of events is similar.

The transition to overshielding after sharp and gradual interplanetary magnetic field northward turning

Overshielding is referred to a shielding status, during which the dawnward shielding electric field dominates over the duskward penetration electric field in the inner magnetosphere, typically appearing when the interplanetary magnetic field (IMF) suddenly turns northward after a prolonged southward orientation. It is expected that the transition to overshielding after IMF northward turning can be affected by the shape of northward turning (sharp or gradual). Moreover, the initial shielding status (undershielding or goodshielding) prior to the transition may also have influence on the transition. Here we analyze two groups of cases, in which the transitions appear after sharp (duration less than 5 min) and gradual (duration more than 30 min) northward turning. Each group includes two cases, in which the transition initiated from undershielding and goodshielding. These cases show that (1) the beginning of the transition to overshielding coincides with sharp IMF northward turning but appears in the midst of gradual IMF northward turning; (2) the transition from goodshielding to overshielding is always associated with convection electric field drop and/or polar cap shrinkage, regardless of the shape of IMF northward turning; and (3) the typical solar wind condition in which the IMF suddenly turns northward after a prolonged southward orientation is neither a necessary condition nor a sufficient condition for overshielding. Furthermore, we will discuss the effect of substorm processes on overshielding.

Interannual and latitudinal variability of the thermosphere density annual harmonics

[1] In this paper we investigate the intra-annual variation in thermosphere neutral density near 400 km using 4 years (2002–2005) of CHAMP measurements. The intra-annual variation, commonly referred to as the ‘‘semiannual variation,’’ is characterized by significant latitude structure, hemispheric asymmetries, and interannual variability. The magnitude of the maximum yearly difference, from the yearly minimum to the yearly maximum, varies by as much as 60% from year to year, and the phases of the minima and maxima also change by 20–40 days from year to year. Each annual harmonic of the intraannual variation, namely, annual, semiannual, terannual and quatra-annual, exhibits a decreasing trend from 2002 through 2005 that is correlated with the decline in solar activity. In addition, some variations in these harmonics are correlated with geomagnetic activity, as represented by the daily mean value of Kp. Recent empirical models of the thermosphere are found to be deficient in capturing most of the latitude dependencies discovered in our data. In addition, the solar flux and geomagnetic activity proxies that we have employed do not capture some latitude and interannual variations detected in our data. It is possible that these variations are partly due to other effects, such as seasonallatitudinal variations in turbopause altitude (and hence O/N2 composition) and ionosphere coupling processes that remain to be discovered in the context of influencing the intraannual variations depicted here. Our results provide a new data set to challenge and validate thermosphere-ionosphere general circulation models that seek to delineate the thermosphere intra-annual variation and to understand the various competing mechanisms that may contribute to its existence and variability. We furthermore suggest that the term ‘‘intra-annual’’ variation be adopted to describe the variability in thermosphere and ionosphere parameters that is well-captured through a superposition of annual, semiannual, terannual, and quatra-annual harmonic terms, and that ‘‘semiannual’’ be used strictly in reference to a pure 6-monthly sinusoidal variation. Moreover, we propose the term ‘‘intraseasonal’’ to refer to those shorter-term variations that arise as residuals from the above Fourier representation.

Constructive interference of large-scale gravity waves excited by interplanetary shock on 29 October 2003: CHAMP observation

In this paper we report the detection of full constructive interference between two large-scale gravity waves in the upper thermosphere from the CHAMP accelerometer measurements. The two waves are separately excited in northern and southern auroral regions by the shock-induced auroral intensification on 29 October 2003. They propagate equatorward and encounter near the equator, where constructive interference occurs and causes nightside equatorial neutral density enhancements of ∼60%. This result demonstrates that the constructive interference can be a potential mechanism for large density increases in the equatorial region during magnetically active periods.

Efficiency of solar wind energy coupling to the ionosphere

We present a statistical investigation into the variations of the ionospheric energy coupling efficiencies with the solar wind energy input, the interplanetary magnetic field (IMF) clock angle and the solar wind dynamic pressure. The ionospheric energy coupling efficiencies are defined as the ratios of the ionospheric energy deposition (namely auroral precipitation, Joule heating, and their total) to the solar wind energy input. We find that the ionospheric energy coupling efficiencies decrease exponentially with the solar wind energy input. Moreover, it is the same case under geomagnetic storm conditions. Our results also show that the energy coupling efficiencies are dependent on the IMF clock angle and almost independent of the solar wind dynamic pressure. These results will help us estimate and predict energy transfer from the solar wind to the thermosphere-ionosphere system under extreme space weather conditions, particularly severe geomagnetic storms.

Annual variations in westward auroral electrojet and substorm occurrence rate during solar cycle 23

The International Monitor for Auroral Geomagnetic Effects network magnetic measurements during the period 1995-2009 are used to characterize the annual variations in the westward electrojet. The results suggest that the annual variations in different local time sectors are quite different due to the different sources. In the MLT sector 2200-0100, the annual variations with maxima in winter suggest they are caused by the combined effects of the convective electric field and the conductivity associated with particle precipitation. Furthermore, the conductivity seems to play a more important role in the MLT sector similar to 2200-2320, while the convective electric field appears to be more important in the MLT sector similar to 2320-0100. In the MLT sector 0300-0600, the annual variations with maxima in summer suggest they are caused by solar EUV conductivity effect and the equinoctial effect. The solar EUV conductivity effect works by increasing ionospheric conductivity and enhancing the westward electrojet in summer, while the equinoctial effect works by decreasing solar wind-magnetosphere coupling efficiency and weakening the westward electrojet in winter. In the MLT sector 0100-0300, the annual variations are relatively weak and can be attributed to the combined effects of annual variations caused by all the previously mentioned effects. In addition, we find that a significant annual variation in substorm occurrence rate, mainly occurring in the premidnight region, is quite similar to that in the westward electrojet. We suggest that elevated solar wind driving during the winter months contributes to higher substorm occurrence in winter in the Northern Hemisphere.

Auroral electrojets variations caused by recurrent high-speed solar wind streams during the extreme solar minimum of 2008

The IMAGE network magnetic measurements are used to investigate the response of the auroral electrojets to the recurrent high-speed solar wind streams (HSSs) during the extreme solar minimum period of 2008. We first compare the global AU/AL indices with the corresponding IU/IL indices determined from the IMAGE magnetometer chain and find that the local IMAGE chain can better monitor the activity in MLT sectors 1230-2230 for IU and 2230-0630 for IL during 2008. In the optimal MLT sectors, the eastward and westward electrojets and their central latitude reveal clear 9-day periodic variations associated with the recurrent HSSs. For the 9-day perturbations, both the eastward and westward electrojet currents are better correlated with parallel electric field (EPAR) and electron hemispheric power (HPe) than with other forcing parameters. Interestingly, the eastward electrojet shows good correlations (r > 0.6) with EPAR and HPe only in part of its optimal MLT-sector, roughly 1200-1800, while the westward electrojet shows good correlations (r < -0.6) with EPAR and HPe in its whole optimal MLT sector. The poor correlations between the eastward electrojet and EPAR and HPe in the MLT sector 1800-2200 might be attributed to the impact of other magnetosphere-ionosphere coupling processes. The sensitivities of the eastward and westward electrojet currents to EPAR are close to 0.06 MA/(mV/m) and -0.12 MA/(mV/m), respectively, and the sensitivities of their central latitudes to EPAR are close to -2.83 Deg/(mV/m) and -2.14 Deg/(mV/m), respectively. The observed auroral electrojet response to the recurrent solar wind forcing provides new opportunities to study the physical processes governing the eastward and westward auroral electrojets.

Observations of a large‐scale gravity wave propagating over an extremely large horizontal distance in the thermosphere

In this paper we report the detection of a large-scale gravity wave propagating over an extremely large horizontal distance in the thermosphere on 28 July 2006. Specifically, after being launched at the northern auroral region on the dayside, this wave propagated equatorward with phase speeds on the order of ∼720 m/s and finally almost traveled around the Earth once horizontally in the thermosphere prior to dissipation. The time taken to dissipate is about 15.5 h. It is the farthest-traveling large-scale gravity wave currently tracked by satellite measurements, made possible by a sudden injection of energy in an unusually clean propagation environment. This experiment of opportunity serves as an important step in furthering our theoretical understanding of gravity wave propagation and dissipation in the thermosphere.

Alfvén waves as a solar-interplanetary driver of the thermospheric disturbances

Alfvén waves have been proposed as an important mechanism for the heating of the Sun’s outer atmosphere and the acceleration of solar wind, but they are generally believed to have no significant impact on the Earth’s upper atmosphere under quiet geomagnetic conditions due to their highly fluctuating nature of interplanetary magnetic field (i.e., intermittent southward magnetic field component). Here we report that a long-duration outward propagating Alfvén wave train carried by a high-speed stream produced continuous (~2 days) and strong (up to ±40%) density disturbances in the Earth’s thermosphere in a way by exciting multiple large-scale gravity waves in auroral regions. The observed ability of Alfvén waves to excite large-scale gravity waves, together with their proved ubiquity in the solar atmosphere and solar wind, suggests that Alfvén waves could be an important solar-interplanetary driver of the global thermospheric disturbances.

Prolonged multiple excitation of large‐scale Traveling Atmospheric Disturbances (TADs) by successive and interacting coronal mass ejections

Successive and interacting coronal mass ejections (CMEs) directed earthward can have significant impacts throughout geospace. While considerable progress has been made in understanding their geomagnetic consequences over the past decade, elucidation of their atmospheric consequences remains a challenge. During 17-19 January 2005, a compound stream formed due to interaction of six successive halo CMEs impacted Earths magnetosphere. In this paper, we report one atmospheric consequence of this impact, namely, the prolonged multiple excitation of large-scale (>approximate to 1000km) traveling atmospheric disturbances (TADs). The TADs were effectively excited in auroral regions by sudden injections of energy due to the intermittent southward magnetic fields within the stream. They propagated toward the equator at speeds near 800m/s and produced long-duration (approximate to 2.5days) continuous large-scale density disturbances of order up 40% in the global thermosphere.