Kazumasa Imai

Modeling Jupiter's decametric modulation lanes

The modulation lanes in Jupiters decametric radio spectra were discovered by Riihimaa [1968]. We have developed a model for the mechanism responsible for their production in which the free parameters have been adjusted to provide a very close fit with the observations. In our model, a grid-like interference screen composed of field-aligned columns of enhanced or depleted plasma density is located near Ios orbit close to the longitude of the sub-Earth point. The column spacing is typically about 140 km. As a band of frequency components emitted from near the foot of an excited tube of magnetic flux passes through the screen, interference patterns of slightly different orientations are produced by the different frequencies. The corotation of this set of interference patterns with Jupiter results in the sloping modulation lanes of the observed dynamic spectrum. Newly calculated results indicate that (1) the Io-B and Io-A radiations are emitted from the northern hemisphere, while that from Io-C comes mainly from the southern hemisphere, (2) the half-angle of the assumed hollow-cone emission beam for Io-B is typically 60°, with a variation of a few degrees, and (3) the equatorial lead angle of the radio-emitting previously excited flux tube ahead of the flux tube through Io at the same instant is more variable, 50° being a typical value for Io-B. Modulation lanes from Io-unrelated emission were also successfully modeled, the assumption in this case being that some source of energy other than a direct encounter with Io excited the flux tube containing the emitting radio source. The L shell value for non-Io-A was less clearly defined but was definitely between 4 and 7. Sensitive measurements of the lane modulation depths for Io-B and Io-A indicated that the decay time for the columns of the interference screen is of the same order of magnitude as Jupiters rotation period. Although the predominant modulation lane periodicity of about 2 s indicates an upper limit on Io-B source width of 70 km along the direction of increasing longitude, finer structure that was present in at least one case suggests an upper limit of only 20 km.

A model for the production of Jupiter's decametric modulation lanes

We present an interference-screen model for the production of the modulation lanes often observed in Jupiters decametric radiation. As radiation traverses a grid-like screen composed of field-aligned columns of enhanced plasma density downstream from Io, rays scattered on passing through the columns interfere constructively and destructively with those passing relatively unscattered between columns. Due to the geometrical relationship between the radio-emitting flux tube and the interference screen, the resulting rotating lobe pattern received at Earth synthesizes modulation lanes that very closely match those actually observed.

Modeling Jovian hectometric attenuation lanes during the Cassini flyby of Jupiter

The Jupiter encounter by the Cassini spacecraft in late 2000 and early 2001 unveiled persistent properties of Jupiters hectometric (HOM) radiation originating along auroral magnetic field lines in the polar regions. One of the unique properties of the HOM dynamic spectrum, known as attenuation lanes, appears as rotationally modulated, well-defined regions of lowered intensity, flanked by regions of enhancement. These lanes seem to be the result of refraction of radio waves in a high-density medium–either caused by Case (i) enhanced density in the magnetic L-shell connected to Ios orbit or Case (ii) in the Io plasma torus itself or both. In this paper, we investigate the HOM ray paths of 0.5–3.0 MHz emissions with various cone half-angles in the continuous radio longitudes generating at the magnetic L-value equal to 30. We use bi-kappa particle distributions to derive diffusive equilibrium distributions of density in the Io plasma torus. The enhanced density irregularities along the Io flux shell “ribbon” region can be described with a Gaussian density distribution of a maximum density n0 and breadth (half-width of the distribution across the flux shell) σ. As a result, we found that the interpretation of Case (i) can be accounted for by the attenuation lanes which appear for all cone half-angles, and the reasonable flux shell density n0 is, on top of specific latitude-dependent density from the diffusive equilibrium model, estimated as 100 cm−3 with the half-width σ = 5.0 Io radii.

Probing Jovian decametric emission with the long wavelength array station 1

New observations of Jupiters decametric radio emissions have been made with the Long Wavelength Array Station 1 (LWA1) which is capable of making high quality observations as low as 11 MHz. Full Stokes parameters were determined for bandwidths of 16 MHz. Here we present the first LWA1 results for the study of six Io-related events at temporal resolutions as fine as 0.25 ms. LWA1 data show excellent spectral detail in Jovian DAM such as simultaneous left hand circular (LHC) and right hand circular (RHC) polarized Io-related arcs and source envelopes, modulation lane features, S-bursts structures, narrow band N-events, and interactions between S-bursts and N-events. The sensitivity of the LWA1 combined with the low radio frequency interference environment allow us to trace the start of the LHC Io-C source region to much earlier CMLIII than typically found in the literature. We find the Io-C starts as early as CMLIII = 230 degrees at frequencies near 11 MHz. This early start of the Io-C emission may be valuable for refining models of the emission mechanism. We also detect modulation lane structures that appear continuous across LHC and RHC emissions, suggesting that both polarizations may originate from the same hemisphere of Jupiter. We present a study of rare S-bursts detected during an Io-D event and show drift rates are consistent with those from other Io-related sources. Finally, S-N burst events are seen in high spectral and temporal resolution and our data strongly support the co-spatial origins of these events.

Development and Results of Compact and Lightweight CanSat Kit “Zero” for Space Technology Education

87 1.序論 缶サットとは,空き缶サイズの模擬人工衛星(satellite) のことである.観測機器を搭載した人工衛星は,地球な どを周回しながら,観測データなどを地球に送り,我々 の生活を支えている.しかし,人工衛星の打ち上げには 多額の費用がかかる.部品も高価でその素材なども慎重 に選ぶ必要があり,学生個人が気軽に製作するのは難し い.そこで宇宙開発などに関わる人材育成のために,地 上での動作を前提とした模擬人工衛星を製作し,衛星開 発の疑似体験をすることがよく行われる.学生が気軽に 缶サット製作を始めるためには,部品やプログラム,製 作手順などをセットにしたキットがあると便利である. 一方で,缶サットの観測実験には,モデルロケットや ハイブリッドロケット,係留気球等が用いられている. 気球係留には,高価なヘリウムガスが大量に必要なため, 少ない数の缶サット投下実験には向いていない.実際の 人工衛星と同じようにロケットによる打ち上げが理想的 であるが,通常の缶サットをある程度の高度に打ち上げ るためには,価格の高いロケットエンジンが必要だった り,火薬取り扱いのための手続きが必要だったりする. 缶サットの性能を保ちつつ,缶サットを小型軽量化する ことができれば,比較的安価で,取り扱いが危険でない モデルロケットでの打ち上げが可能となる. 2015年度の高専スペースキャンプでは,高等専門学校 生(高専生)を対象として宇宙人材育成のため,缶サッ ト講座および缶サット大会を開催した.講座で缶サッ トを製作し,翌日に投下実験を行った.使用したキット (以下,缶サットキット2015と呼称)は,500ml缶の中に マイコンボードとしてRaspberry Pi 2B,気圧センサや webカメラを搭載する単純な構造である. その後,より小型で安価なマイコンボード(Raspberry Pi Zero)の発売開始を受けて,缶サット製作未経験者向 けとして小型化した缶サットキットZeroを開発するこ とにした.図1に,缶サット用に準備した2種類の空き缶 を示す.今回使用した小 型の缶は内容量160gの 缶(φ53mm×91.6mm) で,コーヒー缶よりも 更に短い.大きめの缶 は,缶サットキット2015 でも使用しており,缶サ ット製作にはよく使われ る500ml缶(φ66mm× 166.7mm)である.小型 のマイコンボードを使用 することで,今までの缶 サットに比べて,かなり 小型軽量化されることが 期待される. 2017 年2月 21 日受付 ※1 高知工業高等専門学校 宇宙技術教育のための小型軽量 缶サットキットZero の開発と成果