(xiongchao@whu.edu.cn syma@whu.edu.cn) Nighttime 4-peak Longitudinal Structure of Ionospheric Plasma Density at Mid-Low latitudes During High and Extreme Low Solar Activity Years   C., Xiong (1,2), S.Y., Ma (1), H., Lühr (2) (1). Department of Space Physics, College of Electronic Information, Wuhan University, Wuhan 430079, China (2). Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 14473, Potsdam, Germany (xiongchao@whu.edu.cn syma@whu.edu.cn)

Outline CHAMP and GRACE Ne observation during 23/24 solar cycle. Nighttime 4-peak Longitudinal Structure of Ionospheric Plasma Density. Summary.

1. CHAMP and GRACE Ne observation during 23/24 solar cycle. Electron density in the Earth’s ionosphere, which affects satellite navigation and communications, is highly variable. Solar activity, which increases and decreases over an approximately 11-year cycle, is a key driver of this ionospheric variability. The most recent solar minimum was unusually prolonged, leading to unusual changes in the ionosphere.

Data and Measurements The Planar Langmuir Probe (PLP) on board CHAMP satellite takes in-situ measurements of the electron density every 15 s. The Ne readings of the PLP have been verified by comparison against digisonde measurements at Jicamarca. For the GRACE observation, we make use of the K-band ranging (KBR) system, which measures the dual one-way range change between the two satellites. The total electron content (TEC) between the spacecraft can be deduced from the KBR data. When dividing the horizontal TEC by the distance between the spacecraft we get the average electron density. A more detailed description of the electron density retrieval is given in section 3 of Xiong et al.,[2010].

CHAMP and GRACE Ne observation from 2000 to 2010, orbital averages are averaged over 31 days.

CHAMP orbits decay gradually from 450 km to 330 km over the years, while GRACE stays at about 480 km. We can also see the decline of F10.7, which reaches the minimum in 2009.

2. Nighttime 4-peak Longitudinal Structure of Ionospheric Plasma Density. Recently, growing evidence is provided on longitudinal modulation of ionospheric quantities by tidal effects originating from the tropical troposphere. Such as For example, the four-peaked longitudinal structure in ionospheric UV emission, equatorial electrojet (EEJ) intensity, the total electron content (TEC), the vertical plasma drift and the equatorial ionization anomaly (EIA). All these studies provide a phenomenological relation of the wave-number 4 (WN4) structure to the tide mode component of DE3 (diurnal eastward wave number 3). Longitudinal dependence of the diurnal variation of the EEJ peak current density at March equinox for solar flux, F10.7 = 150. [Luehr, et al., 2008]

Different from the previous studies, which discuss the wave-4 structure of the ionosphere quantities and their compiling with DE3 or other tides component, here, we mainly focus on the nighttime 4-peak longitudinal structure of ionospheric plasma density at mid-low latitudes during high and extreme low solar activity years. As DE3 maximized around August, we have choos the months around August (from 10, June to 20, Oct.) in high (2002, average F10.7=173 sfu) and low (2008, average F10.7=67 sfu) solar activity year .

CHAMP 2002 CHAMP 2008 st sp st sp GRACE 2002 GRACE 2008 st sp st sp

Depletion ward North North ward East Ward Eastward

EPB occurrence rate and the peak-4 of Ne EPB occurrence rate versus season/longitude (left) and local time (right) based on CHAMP PLP observation for the two solar flux activity levels. EPB around -120°E~-30°E “eat” the Ne and cause Ne depletion.

3. Summary (1). In 2002, the equatorial ionization anomaly (EIA) and peak-4 structure of plasma density can be seen around midnight until 0400LT in the morning. While in 2008, EIA and the peak-4 structure of plasma density only last until 22LT. (2). The peak at -90°E deeply decreases around 19LT in 2002 and around 20LT in 2008. One possible reason for the peak decrease at -90°E may be the equatorial plasma bubble (EPB), which cause the electron density depletion. (3). The northward movement of the peak in 2002 may be attributed to the northward neutral wind before midnight around -90°E, which needs further study by the wind model and observations.

Reference: Lühr, H., Rother, M., Häusler, K., Alken, P., and Maus, S.: The influence of non-migrating tides on the longitudinal variation of the equatorial electrojet, J. Geophys. Res., 113, A08313, doi:10.1029/2008JA013064, 2008. Lühr, H., and C. Xiong (2010), IRI‐2007 model overestimates electron density during the 23/24 solar minimum, Geophys. Res. Lett., 37, L23101, doi:10.1029/2010GL045430. Wan, W., L. Liu, X. Pi, M.-L. Zhang, B. Ning, J. Xiong, and F. Ding (2008), Wavenumber-4 patterns of the total electron content over the low latitude ionosphere, Geophys. Res. Lett., 35, L12104, doi:10.1029/2008GL033755. Xiong, C., J. Park, H. Luhr, C. Stolle, and S.Y. Ma (2010), Comparing plasma bubble-occurrence rates at CHAMP and GRACE altitudes during high and low solar activity,Ann. Geophys., 28, 1647-1658.

Thank you all