22 July, 2009 Total Solar Eclipse: Effect on D-region Ionosphere Dynamics as Studied from AWESOME VLF Observations
Rajesh SinghB. Veenadhari, A.K. Maurya
Indian Institute of Geomagnetism
P. Pant: ARIES, Manora Peak, Nainital – IndiaA.K. Singh: Physics Department, B.H.U. , Varanasi – India
Principle Sources of Ion production in D-region Ionosphereduring: Daily usual Sun
There are several sources of ion production for ionospheric D region:
Lyman-alpha line of the solar spectrum at 121.5 nm wavelength penetrates below 95 km and ionize the minor species NO
The EUV radiation between 80.0 and 111.8 nm wavelength and X-rays of 02-0.8 nm wavelength ionize O2 and N2 and thus are the main sources of the free electrons in the ionospheric D region
during: Eclipsed Sun During Total Solar Eclipse, D-region ionosphere of the umbral & penumbral shadow portion of the earth experiences sudden changes.
So solar eclipses provide opportunities to study the physical and chemical processes which determine the behavior of D-region ionosphere
Clilverd et al., 2001: August 11, 1999 Total Solar eclipse effect
• Used both medium and long path VLF signals
• Observed positive amplitude change on path lengths < 2000 km
• Negative amplitude changes on paths > 10,000 km
• Negative phase changes were observed on most paths, independent of path lengths
They further calculated electron concentration values at 77 km altitude throughout the period of solar eclipse, which showed a linear variation in electron production rate with solar ionizing radiation.
40%
40%
Totality at 01:50:00 UT
~ 57 minutes
Totality at 00:53:00 UT
Distance to NWC~ 6700 km
Distance to JJI ~ 4750 km
Indian Stations
to JJI(22.2kHz)
to NWC(19.8kHz)
Totality at ~00:55:00 UT~ 45 seconds Totality at ~00:56:00 UT
3 min 12 seconds
Maximum at ~00:57:00 UT
Two signals - NWC & JJI Two signals - NWC & JJI (1) Intersecting the totality path(1) Intersecting the totality path(2) Along the totality path(2) Along the totality path
to NWC(19.8kHz)
Effect on NWC:Intersecting the Path of Totality at: Allahabad
Allahabad: 25.400 N 81.930 E Eclipse Magnitude = 1 Totality Duration = 45.6 sec
Start of Partial Eclipse - 00:00:17.00Start of Total Eclipse - 00:55:08.9Maximum Eclipse - 00:55:31.4End of Total Eclipse - 00:55:54.3End of Partial Eclipse - 01:56:46.1
(Time in UT)
Decrease in Amplitude of signal as the eclipse progresses Maximum depression around the period of TOTALITY ( ~ 45 sec) A significant decrease in amplitude of 1.5 dB is observed Reaching minimum close to time of totality on the ~ 6700 km path between NWC VLF transmitter and Allahabad Also shift in Morning terminator time is seen from ~ 00:30 UT to time in eclipse totality
to NWC(19.8kHz)
Effect on NWC: Intersecting the Path of Totality at: Varanasi
Varanasi: 25.270 N 82.980 E
Eclipse Magnitude = 1.015TotalityDuration= 3 min 11.5 sec
Start of Partial Eclipse: 00:00:03Start of Total Eclipse: 00:54:08Maximum Eclipse: 00:55:42.6End of Total Eclipse: 00:57:17.1End of Partial Eclipse: 01:56:46
(Time in UT)
Decrease in Amplitude, Minimum depression around the period of TOTALITY
A significant decrease in amplitude of 2.5 dB is observed
Extended period of depression is observed because totality period is ~ 3 min 12 sec
Reaching minimum close to time of totality on the ~ 6700 km path between NWC VLF transmitter and Varanasi
Here again shift in Morning terminator time from ~ 00:30 UT to time in eclipse totality
to NWC(19.8kHz)
Effect on NWC: Intersecting the Path of Totality at: Nainital
Nainital: 29.350 N 79.450 E Eclipse Magnitude = 0.845 NO Totality
Start of Partial Eclipse - 00:03:36Maximum Eclipse - 00:57:18End of Partial Eclipse - 01:56:19
(Time in UT)
First increase in amplitude is seen with the start of eclipse
Then a significant decrease in amplitude of is observed around the time of maximum eclipse
Observations from SID: Bushan, S. Korea
Bushan
Y-Sil Kwak:KASI, Daejeon - South KoreaS Park: KAIST, Bushan - South Korea
By analyzing the dispersive part of tweeks we By analyzing the dispersive part of tweeks we can estimate :can estimate :
A.A. Reflection height (h) of lower region (D-region) of Reflection height (h) of lower region (D-region) of ionosphereionosphere
B.B. Night time Electron density (N)Night time Electron density (N)
C.C. Propagation distance (d) in Earth-Ionosphere Propagation distance (d) in Earth-Ionosphere wave-guide wave-guide
Broadband signals during Total Solar Eclipse: only ONE case
The only example of ionospheric study during eclipse with VLF signal is by Rycroft and Reeve, 1970, Nature, 226, 1126; 1972, JATP, 34, 667
Estimated increase in ionospheric reflection height by 7 km during eclipse of March 7, 1970 from the measurements of tweeks
~ 30 min before Totality
~ 30 min After Totality
~ around Totality
~ around Totality
Tweek Examples during TSE Observed ~ 40 Tweeks
During the total solar eclipse of 22 July 2009 measurements of NWC(19.8 kHz) and JJI(221.2 kHz) VLF transmitter signals where made in India at three sites
Typically negative amplitude changes are seen for the NWC signals whose path intersect the region of totality
SUMMARY
Distance from transmitter to receiver ranged from 6700 km (NWC) & 4750 km (JJI). One path intersecting and other parallel to the movement of totality region
And positive amplitude changes are seen for the JJI signal, which have its propagation path parallel to
The positive and negative changes in amplitude of the VLF signals throughout the whole solar eclipse period shows the changes in D-region ionosphere during eclipse
Further D region ionosphere modeling for earth-ionosphere waveguide propagation NEEDS TO BE DONE to quantitatively infer the information during eclipse period – changes in the ionosphere height, relation between ion production rate and solar ionization, etc..
Broad band observations of Tweek radio atmospherics shows the lower boundary of ionosphere and electron density moving to the levels of night
Importance VLF waves in study of D-region of the Ionosphere
The altitude (~70-90 km) of this region are far too high for balloons and too low for satellites to reach, making continuous monitoring of the ionospheric D region difficult
D-regionD-region is lowest part ofis lowest part of ionosphereionosphere extended fromextended from ~ ~ 50-90 km50-90 km Electron density : ~ 2.5x10Electron density : ~ 2.5x103 el/cc el/cc by dayby day andand decreases to < 10decreases to < 103 3 el/cc el/ccat nightat night
It is generally difficult to measure the ionospheric D region on continuous basis because ionosondes and incoherent scatter radars in the HF-VHF range do not receive echos from this region, where electron density is typically < 103 cm-3
Study of 11 August, 1999 Solar eclipse in Indian Longitude(Sridharan et al., 2002, Ann. Geophy.)
Electrodynamics of the equatorial E- and F- region was studies with observations from ionosondes, VHF and HF radars at Trivandrum
Reported sudden intensification of weak blanketing type Es-layer irregularities, which was pushed down by ~ 8 km during the eclipse.
Because of the fact that VLF waves are almost completely reflected by the D region makes them as a useful tool for studies in this altitude range
Ground based measurements of ELF/VLF waves makes it possible to monitor the state of the D region ionosphere more routinely
Importance VLF waves in study of D-region of the Ionosphere
Carried out both Narrowband and Broadband (Continuous) measurements