Intensive GNSS Radio Occultation Observations by FORMOSAT-7/COSMIC-2 in the Dawn, Noon, Dusk, and Midnight Ionosphere

Po-Han Lee, Jann-Yenq Liu 2, , Chi‐Yen Lin, Fu-Yuan Chang Department of Space Science and Engineering, National Central University, Taoyuan City, TAIWAN Center for Astronautical Physics and Engineering, National Central University, Taoyuan City, TAIWAN Center for Space and Remote Sensing Research, National Central University, Taoyuan City, TAIWAN This manuscript is submitted to “Terrestrial, Atmospheric and Oceanic Sciences” Running title: Intensive GNSS Radio Occultations of FORMOSAT-7/COSMIC-2


INTRODUCTION
The FORMOSAT-7/COSMIC-2 (F7/C2) satellites were launched by SpaceX Falcon Heavy rocket on 25 June 2019, at Kennedy Space Center, Florida, USA (https://www.nspo.narl.org.tw/inprogress.php?c=20022301&ln=en). All six satellites of the F7/C2 constellation were first parked at a 720-km circular orbit during a 1-month period of 25 June-24 July 2019. These satellites were gradually transferred to their mission orbits at 550 km altitude with an inclination angle of 24 degrees and an orbital period of about 97 minutes from 24 July 2019 to 31 January 2021. F7/C2 is equipped with a GNSS receiver, which can receive GPS signals from the United States and GLONASS signals from Russia. By measuring radio occultation (RO) signals, atmospheric temperature and humidity, as well as ionospheric total electron content (TEC) can be derived. The Abel inversion (Hajj and Romans 1998) has been employed to invert and the electron density (Ne) profiles from the RO TEC profiles. These TEC and Ne profiles allow us to have a better understanding of three-dimensional (3D) structures and dynamics of ionospheric plasma (Lin et al. 2007; Liu et al. 2010;Lee et al. 2011;Chang et al. 2015Chang et al. , 2020. During this intensive observation period, simultaneous GNSS RO profiles sounded by 5-6 next adjacent F7/C2 satellites provide a oncein-a-lifetime opportunity to study fine structures of 3D Ne in the low-latitude ionosphere with high temporal resolution. The temporal and spatial sounding ranges around the F2 peak of the F7/C2 satellites are about 5-7 minutes and 40-190 km. In total, 173 events have been observed during the period ( Figure 1). Here, we report fine 3D Ne structures and dynamics at dawn, noon, dusk, and midnight by means of the F7/C2 incursive observations.

OBSERVATION AND RESULT
There are 173 intensive observation events occurring in the 1-month intense observation period of 25 June-24 July 2019 (F10.7: 68-70 sfu, solar flux unit, a convenient measure of spectral flux density often used in solar radio observations.) (Figure 1). We select five events at four-time sectors at dawn, noon, dusk, and midnight to study the response of Ne to sunrise, quasi-equilibrium distribution, pre-reversal enhancement (PRE), and plasma transportations/irregularity (Ratcliffe 1972;Farley et al. 1986;Davies 1990 2).
The six satellites descending southward observe 3D Ne structures ( Figure 3c). Figures 3d and 3e depict that NmF2 increases from 1.02 × 10 5 to 1.33 × 10 5 #/cm 3 , while hmF2 descends from 226.9 to 221.8 km within a horizontal range of 40.9 km during a 455-second sounding duration. Figure 3d illustrates that the NmF2 of F7/C2 increases smoothly (black dashed curve), which is similar to that of F3/C (grey dashed curve), and comparing to a smoothly descending of the F3/C hmF2 (grey solid curve), the hmF2 of F7/C2 yields a descending trend with small structures (black solid curve).
Assuming the ionosphere to be horizontally stratified, we can compute the time rate of electron density changes ( ∂Ne/ ∂t) at a certain altitude during the sounding period (Appendix A). Figure 3f < A c c e p t e d M a n u s c r i p t > Page 5 in 26 pages shows that Ne prominently increases between 200-300 km altitude, especially around the F2-peak height, but slightly increases above 400 km altitude. The percentage of the change to its ambient electron density is a small value, which varies from 0.007 to 0.06%.
To assess the midday effect, we study TEC and Ne profiles sounded by F7/C2 satellite #5, #2, altitude. F7/C2 satellite #5, #2, #4, #1, and #3 observe that below 200 km altitude, the TEC increase correspondingly; however, the Ne oppositely decrease. Therefore, the time variation of electron density is negative below ~250 km, which could be due to the retrieval error of the Abel inversion.
Nevertheless, the percentage of the change to its ambient electron density is a small value of about 0, which confirms the whole ionosphere is right under the quasi-equilibrium distribution.
To study the PRE signatures, events before and after the local sunset are selected. During the early PRE phase, Figure Figure 5f shows that Ne rapidly decreases between 200-350 km altitude, especially below the F2-peak height, and however no obvious Ne temporal changes can be found above 400 km altitude. The percentages of the change to the ambient electron density are small negative values, varying from -0.11 to -0.002%.
For the later PRE phase, Figure 6 illustrates the TEC and Ne profiles observed at 7. The ionosphere is formed in the Earth's upper atmosphere by incident solar radiation interacting with, and removing electrons, from different gases. The time rate of electron density changes is a function of the production rate (Q), the loss rate (L), and the transport ∇ • ( V ⃗ ⃗ ), which can be expressed as (Ratcliffe 1972;Davies 1990), where Q and L are the photochemical processes mainly due to solar ionizations and ion-electron recombination, respectively. Therefore, the events at dawn, noon, dusk, and midnight are used to examine the response of the ionosphere to rapid increase of solar radiation, quasi-equilibrium distribution, rapid decrease of solar radiation together with PRE, and downward diffusions from the magnetosphere, respectively.
Well agreements between F7/C2 and F3/C observations show that due to rapid solar radiation increases NmF2 monotonically increase but hmF2 descends right before the local sunrise (Figures 2   and 3d). Again, owing to the rapid increase in solar radiations, the time rate of electron density changes in the whole ionosphere are all positive (Figure 3f), which indicates that Ne of the whole ionosphere increases. Based on the Chapman distribution (Chapman 1931), the height and the magnitude of the Q maximum are functions of the solar zenith angle. The larger zenith angle results in the higher altitude and the lower rate of the Q maximum. Around the midday, where the zenith angle is small, the height of the Q maximum generally is around 120 km altitude, and therefore it requires strong upward diffusions to transport the electron to a higher altitude, such as the F-region about 300 km altitude (Ratcliffe 1972;Davies 1989). The event is observed 10 minutes before the local sunrise on the ground, where the zenith angle is greater than 90°. Although it is before sunrise, the topside ionosphere might have already experienced solar radiations (Rishbeth & Setty 1960).
Because solar radiations shine the ionosphere from the top to down, even without any strong upward

< A c c e p t e d M a n u s c r i p t >
diffusions, the electron density increases in the whole ionosphere, especially around 200-300 km altitude (Figure 3f).
During the day, as the sun rises and sets, the rate of production by the solar radiation at any level increases to a maximum and then decreases: at some stage the resulting electron density also reaches a maximum, so that = 0, which is termed the quasi-equilibrium distribution. Thus, an event around noon provides a good chance to study the quasi-equilibrium distribution, such as Owing to the eastward and westward electric fields increasing right before and after about the sunset, vertical plasma velocities at the early and late phase of PRE are upward and downward, respectively (Farley et al. 1986). These result in that the ionosphere ascends and the electron density in the lower ionosphere decreases during the PRE early phase, and the ionosphere descends during the PRE late phase. Figures 5d and 5e show that F7/C2 hmF2 drastically ascends and F7/C2 NmF2 significantly decreases in the PRE early phase, which are much more intense than those of F3/C. Figure 5f shows that the time rate of electron density changes at all the altitudes are in negative, which indicates the electron density decreases in the whole ionosphere during the PRE early phase. For the PRE late phase, owing to the downward plasma flows, hmF2 quickly descends and NmF2 rapidly decreases (Figures 6d and 6e). Since there are no solar radiations and the loss process becomes essential after the sunset, the electron density continuously decreases in all altitudes, except that below 250 km altitude, where receive the downward plasma diffusion. Again, tendencies of NmF2 and hmF2 of F7/C2 and F3/C are similar, except the F3/C being less intense.

< A c c e p t e d M a n u s c r i p t >
Since no solar radiations, the loss process in the low ionosphere and downward plasma flows from the magnetosphere become dominant during midnight. F3/C observations depict that in general, hmF2 can shortly vary from ascending to descending, while NmF2 decreases prior to midnight ( Figure 2). Figures 7d and 7e show that for the F7/C2 intensive midnight observation, NmF2 decreases and hmF2 ascends. Figure 7f reveals that due to the loss process, the electron density of the ionosphere generally decreases slightly.
Based on a large amount of data of 47,642 profiles within ±7.5° magnetic latitude observed in the long-term period of June-August during 2006-2010 (1000 days) observed by F3/C, the diurnal variation in the median value of hmF2 and NmF2 along the magnetic equator can be obtained.
Although such a large amount of data has been used, it still has difficulty obtaining the 3D structure of the ionospheric electron density in detail. By contrast, the F7/C2 intensive observation allows us to examine 3D electron density structure and dynamics with very high time and fine spatial resolution during a very short time interval (i.e., event duration) of about 300-500 sec. Tendencies and variations of hmF2 and NmF2 of F7/C2 in the five events generally agree well with those of the F3/C reference, but the former is much more intense than the latter. This is because the former ones are obtained by a signal event of the intensive observation, while the latter ones are the medians calculated from the large amount of data. In conclusion, based on the results of the F7/C2 intensive observation, it is worthy of having a mission with several next adjacent satellites to observe fine plasma structures and dynamics of the ionosphere. providing FORMOSAT-7/COSMIC-2 and FORMOSAT-3/COSMIC data

Tables
Insert tables here. 1 page for 1 table. < A c c e p t e d M a n u s c r i p t >