VHF Lightning Observations by Digital Interferometry from ISS / JEM-GLIMS

Global Lightning and sprIte MeasurementS (GLIMS) mission is now ongoing on Exposed Facility of Japanese Experiment Module (JEM-EF) of the International Space Station (ISS). This paper focuses on an electromagnetic (EM) payload of JEM-GLIMS mission, very high frequency (VHF) broadband digital InTerFerometer (VITF). JEM-GLIMS mission is designed to conduct comprehensive observations with both EM and optical payloads for lightning activities and related transient luminous events (TLEs) expecting to give us many scientific impacts to the field. Its nominal operation is continued from December 2012. Through the operation period, VITF corrects numerous VHF EM data synchronized with optical signals. About 650 VITF datasets were obtained in January and February 2013, for instance. The estimations of the EM direction-of-arrival (DOA) are attempted using the broadband digital interferometry. Some results agree with the optical observations, even though DOA estimation has difficulties caused by its very short baseline of the antennas and multiple pulses in short time, namely burst-type EM waveforms. The recorded VHF EM signals, the results of their DOA estimations and the comparisons with optical observations are introduced. INTRODUCTION The efficacy of global monitoring for environment from space has been demonstrated since 60’s. Many earth observation satellites have been launched, and outstanding contributions are provided. As previous works for the satellite observations associated with lightning, Optical Transient Detector (OTD) on Microlab-1 satellite launched in 1995 and Lightning Imaging Sensor (LIS) on Tropical Rainfall Measuring Mission (TRMM) satellite in 1997 revealed the global distribution of lightning activities with optical observations [Christian et al. 2003; Boccippio et al. 2000]. Array of Low-Energy X-Ray Imaging Sensors (ALEXIS) satellite and Fast On-orbit Recording of Transient Events (FORTE) satellite were launched in 1993 and 1997, respectively. They recorded many transionospheric pulse pair (TIPP) waveforms by the electromagnetic (EM) radio observations [Jacobson et al. 1999; Tierney et al. 2002]. ISS-b satellite, which ∗ Contact information: Takeshi Morimoto, Kinki University, 3-4-1 Kowakae, Higashiosaka City, Osaka, Japan, Email: [email protected] XV International Conference on Atmospheric Electricity, 15-20 June 2014, Norman, Oklahoma, U.S.A. 2 had been launched in 1978 and measured short wave EM noise, could be said the first EM lightning observations [Kotaki et al. 1984]. Maido-1 satellite [Nakamura and Hashimoto 2005] is the world's first specialized satellite for lightning observation that is the ancestor project of this paper. The authors have been developing the ground-based very high frequency (VHF) broadband Digital InTerFerometer (DITF) to image precise lightning channels and to monitor lightning activities widely. The remarkable feature of DITF is its ultra-wide bandwidth (from 25MHz to 100MHz) and implicit redundancy for estimating VHF source locations [Mardiana et al. 2000; Morimoto et al. 2004]. We consider an application of DITF to the space-borne measurement system, and joins Maido-1 satellite of Space Oriented Higashi-Osaka Leading Associate (SOHLA), and Global Lightning and splIte MeasurementS (GLIMS) on Exposed Facility (EF) of Japanese Experiment Module (JEM) at the International Space Station (ISS) projects as mission scientists. The both missions intend to have lightning observation from space by detecting VHF broadband EM signals associated with lightning discharges as a gradual approach to realize space-borne lightning monitoring by means of EM observations [Morimoto et al. 2011]. This paper introduces an EM payload of JEM-GLIMS mission, VHF broadband digital InTerFerometer (VITF) and its observation results.