Characteristics of the Mean Polar Electron Content with UT

Wu Yewen*1,2, Liu Ruiyuan2, Zhang Beichen2, Lv Jianyong1

1. Institute of Space Weather, Nanjing University of Information Science &Technology, Nanjing,210044, China,

ywwu@nuist.edu.cn, jylu@nuist.edu.cn

2. Polar Research Institute of China,Shanghai, 200136, China, ryliu@pric.gov.cn, zhangbeichen@pric.gov.cn

Abstract: We proposed a new ionospheric parameter, mean Polar Electron Content (mPEC), based on the COSMIC data. The variations of mPEC with UT were investigated in the polar region during low solar activity using geographic coordinates system and corrected geomagnetic latitude and magnetic local time coordinates system (CGLMLT), respectively. It can be found that the UT variation is stronger in CGLMLT than geographic coordinates system; the UT variation is stronger in Antarctic than Arctic. Based on the variations of the UT variation of mPEC, the proportions of the electrons produced by solar radiation and the transport and particles precipitation (TP) are investigated. The solar radiation is important than the other two. The TP is relatively important in Antarctic than in Arctic. The reason for the UT variations of the ionosphere in the polar region is the separation of the geographic pole with the geomagnetic pole. The difference between the Antarctic and Arctic because that the separation of these two pole is lager in Antarctic than that in Arctic.

1. Introduction In the polar region, the magnetosphere, ionosphere and thermosphere couples intensively. The interrelated dynamic, energetic, and chemical processes have been believed to be responsible for the complex morphology of the high-latitude F region [1,2]. The ionospheric parameters, such as NmF2, hmF2, change with the angle between the geomagnetic axis and the Earth’s rotation axis [3-5]. Because of the angle, there are obvious UT (Universal Time) variations in the polar region [6-7], which are researched based on the observation of ionosonds in the Antarctic [8,9]. However, the distribution of the ionosonde is not enough dense in the polar region, especially in Arctic.Therefore, the details of the UT variations in deep are not done. In addition, the main processes changing the electron density are solar radiation, transform and particle precipitation in the polar regions. For the question that which one is more important, the models seem to be the only way. Here, based on the electron density observation from COSMIC radio occultation, a new parameter, mean Polar Electron Content (mPEC), is proposed, which is similar with the global mean TEC [10,11]. Therefore, we can analyse the ionospheric UT variations in details in the polar regions. At the same time, we can also estimate the relative importance of the three processes above from the UT variation intensity of mPEC. The results above will be helpful to the numerical analyzing and modeling the polar ionosphere.

2. Method and Data The universal variation of geophysical characteristics such as foF2 is defined as the diurnal variation of the instantaneous mean-longitudinal value of this characteristic with UT for a fixed latitude [7]. Considering the global mean TEC [12-13], the mPEC is defined as follows:

, , ,,

,,

i j UT i ji j

UTi j

i j

TEC S

mPECS

×

=∑

∑ (1)

Here, the Si,j and the TECi,j,UT are the area of the grid (i, j) and its corresponding TEC value, respectively. The UT value is between 0 and 23. The grids are polar region divided by every 2.5 degree latitude and 1LT (Local Time) in the geographic coordinates system and by every 2.5 degree magnetic latitude and 1MLT (Magnetic Local Time) in the corrected geomagnetic latitude and magnetic local time coordinates system (CGLMLT), respectively. The polar region is the region whose latitude is larger than 60 degree. The TEC value is calculated by integration the electron density profile observed by COSMIC radio occultation, whose height is up to 1000 km by exponential extrapolation with the formula (2):

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0( ) exp[ ( )]Ne h A c h h= × × − (2) Where, A is the electron density (Ne) at the height of h0, which is the largest height of one observation of the electron density profile. The c is a parameter by fitting the top ionospheric Ne. The Ne data observed by COSMIC is from 2007 to 2010. There are about 400,000 Ne profiles left after a process of elimination. The data of 4 years are divided into four seasons. In order to make sure enough observation in one grid, the TEC at one UT is the mean value of the TEC from (UT+1) to (UT-1) in the grid.

3. Results The UT variation of mPEC in four seasons in geographic coordinates system is shown in Fig. 1 (left). The red lines and blue lines are Arctic and Antarctic, respectively. The dotted lines are the mean value of the corresponding mPEC. It can be seen that there are obvious UT variation in all four seasons in the Antarctic, while in the Arctic, it is clear in winter but not in the other seasons. The time of the maximum mPEC is also different. It is about 5UT in Antarctic while 16UT in Arctic. The difference, about 12 hours, likes a kind of phase difference between Antarctic and Arctic. In addition, the intensity of the UT variation is stronger in Antarctic than in Arctic. The UT variation of mPEC in four seasons in CGLMLT is shown in Fig. 2. The main characters of the UT variation of mPEC are almost the same as what is shown in Fig. 1, except that there are obvious UT variations in all four seasons in Arctic.

Fig.1. the UT variation of mPEC in four seasons in the Arctic and Antarctic (left, geographic coordinates system, right, CGLMLT)

4. Discussion

4.1 The reasons for the UT variation of mPEC Firstly, in geographic coordinates system. As example, the TEC map at 4UT and 16UT in winter are used to explain that in the Antarctic as shown in Fig. 2, where the red square is the magnetic axis. Generally speaking, the TEC at 4UT is larger than that at 16UT, especially in the dayside. It can also be seen that the axis is closer to the sun than that at 16UT. As is well-known, for the whole polar region, the irradiation of the sun keeps constant in geographic coordinates system, which means that the sun has nothing to do with the variation of TEC. While with the changing of the magnetic pole with UT, the position of convection varies in the polar region in the geographic coordinates system. When the convection is closer to the sun, more electrons in the dayside can be transported to the polar area. Therefore, the TEC is larger when the magnetic axis closer to the sun, then the mPEC becomes larger naturally. In the Arctic, It is the same as that in the Antarctic. Considering the pages, they are not shown here. Secondly, in CGLMLT. The same as above, there are TEC maps chosen as shown in Fig. 3. The red circle is the geographic pole. It can be seen that the TEC is larger when the geographic pole is away from the sun. It is because that the sun can irradiate more polar area in CGLMLT, while the position of the convention and auroral oval seem to be fixed. Therefore, there are more electron are ionized by sun radiation, especially in the dayside. Meanwhile, the convection can transport more the electron to the polar region. Therefore, the TEC is larger when the geographic axis is away from the sun, then the mPEC becomes larger. In the Arctic, It is the same as that in the Antarctic. Considering the pages, they are not shown here, too.

Fig.2. the TEC map at 4UT (left) and 16UT (right) in winter in the Antarctic in geographic coordinates system (Unit: TECu)

Fig.3. the TEC map at 4UT (left) and 16UT (right) in winter in the Antarctic in CGLMLT (Unit: TECu)

4.2 The estimation of the relative importance of Sun and TP Based on the CV of mPEC, the proportions of the electrons produced by solar radiation (Sun) and the transport and particles precipitation (TP) are investigated, too. They are written as Rsun and RTP, and calculated as formula (3) and (4), respectively.

magSun

mag geo

CVR

CV CV=

+ (3)

geoTP

mag geo

CVR

CV CV=

+ (4)

The results are shown in Table1. It can be seen that the solar radiationaccount 60-70% in Antarctic and 75% in winter and 87% in the other seasons in Arctic, which means that the solar radiation is important than the other two. It also can be seen that the transport and particles precipitation (TP) is relatively important in Antarctic than in Arctic. In Antarctic, the TP accounts about 50 for the solar radiation, while in the Arctic, the proportion is about 33% in winter while less than 15% in the other seasons. Table 1. the proportions of the Sun and TP accounts for the UT variation of mPEC

Spring Summer Autumn Winter South North South North South North South North

RTP 0.36 0.13 0.40 0.12 0.33 0.13 0.31 0.25 RSun 0.64 0.87 0.60 0.88 0.67 0.87 0.69 0.75

RTP/RSun 56.4% 14.8% 67.3% 13.7% 49.6% 14.6% 45.0% 32.5% The reason for the UT variations of the ionosphere in the polar region is the separation of the geographic pole with the geomagnetic pole. The difference between the Antarctic and Arctic because that the separation of these two pole is lager in Antarctic than that in Arctic.

4. Conclusions A new parameters, mean Polar Electron Content (mPEC), was proposed based on the observation of COSMIC. It seems to be useful to analyze the UT variation of the ionosphere in the polar region, and also to estimate the relative importance for the electron density between the solar radiation and the transport and particles precipitation. The major results are as follows: 1. In the Antarctic, there are obvious UT variations of the ionosphere in four seasons both in geographic coordinates system and in the corrected geomagnetic latitude and magnetic local time coordinates system (CGLMLT).

The maximum and minimum values of mPEC are about 5UT and 16UT in the Antarctic, respectively. While in the Arctic, UT variations of the ionosphere exist clearly in four seasons in CGLMLT and only winter in geographic coordinates system. The maximum and minimum values of mPEC are about 18UT and 5UT, respectively. 2. It can be found that the UT variation is stronger in CGLMLT than geographic coordinates system; the UT variation is stronger in Antarctic than Arctic. 3. For the electron density, the solar radiation is important than the transport and particles precipitation (TP), which account 60-70% in Antarctic and 75% in winter and 87% in the other seasons in Arctic. The TP is relatively important in Antarctic than in Arctic. In Antarctic, the TP accounts about 50% for the solar radiation, while in the Arctic, the proportion is about 33% in winter while less than 15% in the other seasons. 4. The reason for the UT variations of the ionosphere in the polar region is the separation of the geographic pole with the geomagnetic pole. The difference between the Antarctic and Arctic because that the separation of these two pole is lager in Antarctic than that in Arctic.

Acknowledgments

This work was supported by the National Basic Research Program of China (grant 2012CB825603) and the National Natural Science Foundation of China (grants 41274148). The authors thank UCAR for providing the data of COAMIC radio occultation.

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