Global thermospheric disturbances induced by a solar flare: a modeling study
© Le et al.; licensee Springer. 2015
Received: 5 September 2014
Accepted: 1 December 2014
Published: 8 January 2015
This study focuses on the global thermosphere disturbances during a solar flare by a theoretical model of thermosphere and ionosphere. The simulated results show significant enhancements in thermospheric density and temperature in dayside hemisphere. The greatest thermospheric response occurs at the subsolar point, which shows the important effect of solar zenith angle. The results show that there are also significant enhancements in nightside hemisphere. The sudden heating due to the solar flare disturbs the global thermosphere circulation, which results in the significant change in horizontal wind. There is a significant convergence process to the antisolar point and thus the strong disturbances in the nightside occur at the antisolar point. The peak enhancements of the neutral density around antisolar point occur at about 4 h after solar flare onset. The simulated results show that thermospheric response to a solar flare mainly depends on the total integrated energy into the thermosphere, not the peak value of EUV flux. The simulated results are basically consistent with the observations derived from the CHAMP satellite, which verified the results of this modeling study.
Solar flares produce great enhancements in extreme ultaviolet (EUV) and X-ray radiations, which cause sudden and intense disturbances in the Earth’s upper atmosphere. Ionospheric effects of solar flares, or sudden ionospheric disturbances (SID), have been studied since 1960s owing to their effects on radio communications and navigation systems. Most previous studies related to solar flares have so far focused on the ionospheric responses (e.g., Afraimovich 2000; Leonovich et al. 2002; Liu et al. 2004, 2006; Mahajan et al. 2010; Tsurutani 2005; Wan et al. 2005; Zhang et al. 2002; Zhang and Xiao 2005; Le et al. 2007, 2011, 2013; Liu et al. 2011). Compared to the research on the ionospheric responses to solar flares, the study on the thermospheric responses to solar flares is relatively scarce. It may be due to the much less observation of the thermosphere than that of the ionosphere. The observations of the terrestrial effects of solar flares have recently been extended to the neutral atmosphere. The neutral density data near 400 km has been obtained from accelerometer measurements of nongravitational accelerations on the Challenging Minisatellite Payload (CHAMP) and the Gravity Recovery and Climate Experiment (GRACE) satellites.
Some studies have been performed to quantify the thermospheric response to flares based on the neutral density data from the CHAMP and GRACE satellites (e.g., Forbes et al. 2005; Sutton et al. 2006; Liu et al. 2007; Pawlowski and Ridley 2008, 2009, 2011). These studies show that in addition to the great disturbances in the ionosphere, solar flares can also induce significant responses in the thermosphere. Sutton et al. (2006) reported the first measurements of the thermosphere density response to two great flare events on 28 October 2003 (X17) and 4 November 2003 (X28). They found the thermosphere density increases associated with the flares are about 50% to 60% and 35% to 45%, respectively, at low to mid-latitudes. Liu et al. (2007) examined both the thermospheric and ionospheric responses to the solar flare on 28 October 2003 by using measurements from the CHAMP satellite. Pawlowski and Ridley (2008) simulated the thermospheric responses to the solar flare on 28 October 2003 using the Global Ionosphere-Thermosphere Model (GITM). Their results show that the thermospheric density at 400 km can increase by as much as 14.6% in about 2 h and takes 12 h to return back to a normal state. Qian et al. (2010) also investigated the ionosphere and the thermosphere response to the X17 solar flare on 28 October 2003 by the thermosphere-ionosphere-mesosphere electrodynamics general circulation model (TIME-GCM). The results also show the significant thermospheric disturbances including about a maximum increase of 20% in neutral density at altitude of 400 km.
As mentioned above, there are many studies focusing on ionospheric and thermospheric responses to the great X17 solar flare on 28 October 2003. How about a weaker flare’s effect on the thermosphere? Le et al. (2012) statistically analyzed the neutral density variation for all X-class solar flares during 2001 to 2006 based on the density data derived from accelerometers on the CHAMP and GRACE satellites. The observed results show the density response for X1 to X5 flares falls within the noise level in the thermosphere, but there is significant neutral density response for X5 and stronger flares, with an average enhancement of 10% ~ 13% in neutral density at the altitude of 400 km at mid-low latitudes within about 4 h after solar flare onset. In this study, we would explore the global thermospheric disturbances induced by a solar flare with magnitude of X5 and the transport process of energy by using the Global Coupled Ionosphere-Thermosphere-Electrodynamics Model, developed by the Institute of Geology and Geophysics, Chinese Academy of Sciences (GCITEM).
GCITEM model introduction
The model employed in this study is the GCITEM model (Ren et al. 2009). It is a new global 3-D self-consistent model of the ionosphere and thermosphere including electrodynamics. This new model self-consistently calculates the time-dependent 3-D structures of the main thermospheric and ionospheric parameters in the height range from 90 to 600 km, including neutral number density of the major species O2, N2, and O and the minor species N(2D), N(4S), NO, He, and Ar; ion number densities of O+, O2 +, N2 +, NO+, N+, and electron; neutral, electron, and ion temperature; and neutral wind vectors. The ionospheric electric fields in the mid and low latitudes can also be self-consistently calculated. The model is based on the hydrostatic assumption. It is a full 3-D code with 5° latitude by 7.5° longitude grids in a spherical geomagnetic coordinate system. The vertical grid spacing is about 3 km in the lower thermosphere and about 30 km in the upper thermosphere. This model is solved by a time-stepping finite difference procedure, with a time step of 2 ~ 5 min. The horizontal difference procedure uses explicit numerical method. To get the rapid vertical molecular diffusion, the vertical difference procedure uses implicit numerical method. The GCITEM model can simulate the ionosphere-thermosphere system in a realistic geomagnetic field. This is important in the research of the couple between the neutrals and ions, and the anomaly of the ionospheric and thermospheric spatial-temporal variations. GCITEM-IGGCAS can simulate the complex chemistry, thermodynamics, dynamics, and electrodynamics of the coupled ionosphere-thermosphere system. Thus, it is an effective platform to simulate solar flare effects on the thermosphere and ionosphere. In this study, we focus on the thermospheric responses to a solar flare.
The reference simulation is carried out on 6 April (day of year = 96) with the median solar activity condition (F10.7 = 140.0). Then the solar EUV variation during the solar flare is embedded in the GCITEM model to model the thermospheric responses to the solar flare. The time step in the simulations is 2 min. The solar flare onset is set at 0100 UT. The differences of neutral density, neutral temperature, and horizontal wind between the two simulations are calculated to represent the responses of density, temperature, and wind.
Results and discussion
It is well known that the ionospheric response to a solar flare is very fast because of the small time constant of electron and ions. The time and amplitude of the peak response are determined by peak solar irradiance (e.g., Afraimovich 2000; Zhang et al. 2002, 2011; Le et al. 2007). However, because of the large mass and high heat capacity in the thermosphere, the neutral gas response to a solar flare would be sluggish with regard to transient enhancement in solar EUV flux; that is, it needs longer time for neutral gas to react to the increase in solar EUV flux, and the neutral gas response also can last a longer time. Our simulations just show such a feature. For example, the percentage change of neutral density at subsolar point reaches the peak at Δt ≈ 1 h when the EUV flux return to the normal level, and it takes as long as more than 20 h to return back to the background level.
Figure 12b, c shows the dayside average responses of neutral density and neutral temperature to the three solar flares: half duration, normal duration, and double duration. The peak enhancement in the neutral density is 21.7%, 13.7%, and 7.1% for the double duration flare, the normal flare, and the half duration flare, respectively. The peak increase in neutral temperature is 64.5, 42.8, and 22.2 K for the double duration flare, the normal flare, and the half duration flare, respectively. The results suggest that although with the same peak EUV flux, the larger integrated energy being deposited into the atmosphere results in the larger thermospheric responses. Pawlowski and Ridley (2011) reported a linear dependence of neutral density response on the total integrated energy. Our simulations show that is not a rigorous linear dependence between the density response and the total integrated energy. The peak enhancement in the neutral density does not increase by 100% but only increase by approximately 58% when the flare duration prolongs by 100% from the normal condition to double duration. These results show that the enhancement in neutral density increases nonlinearly with integral EUV flux. Its amplification decreases with integral EUV flux.
Figure 12 (normal duration lines) also illustrates the dayside average disturbance level of thermospheric density and temperature at 400 km. The results show that there is a peak enhancement of about 13.7% in the global average neutral density at 400 km and a peak increase of about 42 K in the global average neutral temperature at 400 km. It takes about 20 h for the density enhancement to drop to 1% and takes about 20 h for the temperature increase to drop to 2 K. These results show that the thermospheric responses to a solar flare can last more than 20 h, which is much longer than the ionospheric responses of which the typical duration is less than 2 h (Tsurutani 2005; Le et al. 2007).
The thermospheric responses to a solar flare were investigated, based on the results of a modeling effort by the GCITEM model. The solar EUV variation during a solar flare is derived from the empirical model FISM. In this study, we did not model the thermospheric response to the great solar flare like the X17 solar flare on 28 October 2003, but modeled the effects of a weaker flare of X5. The simulated results show that there are significant enhancements in the neutral density and neutral temperature. The strongest responses of the thermosphere occur at the subsolar point. The larger solar zenith angle causes the smaller thermospheric responses, which suggests the strong effect of the solar zenith angle. The peak responses appear about 1 h after the solar flare onset for all latitudes. The thermospheric responses to a solar flare can last more than 20 h.
In addition to the significant disturbances in the dayside, the simulated results also show that a solar flare can produce significant disturbance in the nightside. The percentage change of the neutral density in the nightside is just a little smaller than that in the dayside. There is a significant latitude dependence for nightside thermospheric response. It almost synchronizes with dayside in the polar region and high latitudes but is later at lower latitudes. The peak disturbance in the nightside appears at the antisolar point. The heating from the sudden increases in solar radiation during a solar flare disturbs the global thermosphere circulation, which results in the significant change in the horizontal wind. There is significant convergence process to the antisolar point, and thus the strong disturbances in the nightside occur at the antisolar point. The peak enhancement in the nightside appears about 4 h after the solar flare onset. In addition, our simulations show that although with the same peak EUV flux, the solar flare with longer duration would produce the stronger thermospheric response, which suggests that the integral energy during a solar flare is an important factor of the thermospheric responses to a solar flare. To verify quantitatively the result of this modeling study, the simulated results are compared with the observations of neutral density derived from the accelerometers on the CHAMP satellite. The simulated results are basically consistent with the observed results, although the observed results have larger latitudinal fluctuation.
This research was supported by the Chinese Academy of Sciences project (KZZD-EW-01-3), National Key Basic Research Program of China (2012CB825604), National Natural Science Foundation of China (41374162, 41231065, and 41321003).
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