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Optical and Radio Solar Observation for Space Weather

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Optical and Radio Solar Observation for Space Weather 2 Solar and Solar wind 2-1 Optical and Radio Solar Observation for Space Weather AKIOKA Maki, KONDO Tetsuro, SAGAWA Eiichi, KUBO Yuuki, and IWAI Hironori Researches on solar observation technique and data utilization are important issues of space weather forecasting program. Hiraiso Solar Observatory is a facility for R & D for solar observation and routine solar observation for CRL's space environment information service. High definition H alpha solar telescope is an optical telescope with very narrow pass-band filter for high resolution full-disk imaging and doppler mapping of upper atmos- phere dynamics. Hiraiso RAdio-Spectrograph (HiRAS) provides information on coronal shock wave and particle acceleration in the soar atmosphere. These information are impor- tant for daily space weather forecasting and alert. In this article, high definition H alpha solar telescope and radio spectrograph system are briefly introduced. Keywords Space weather forecast, Solar observation, Solar activity 1 Introduction X-ray and ultraviolet radiation resulting from solar activities and solar flares cause distur- 1.1 The Sun as the Origin of Space bances in the ionosphere and the upper atmos- Environment Disturbances phere, which in turn cause communication dis- The space environment disturbance phe- ruptions and affect the density structure of the nomena studied in space weather forecasting Earth's atmosphere. CME induce geomagnet- at CRL include a wide range of phenomena ic storms and ionospheric disturbances upon such as solar energetic particle (SEP) events, reaching the Earth's magnetosphere, and are geomagnetic storms, ionospheric disturbances, believed to be responsible for particle acceler- and radiation belt activity. All of these space environment disturbance events are believed to have solar origins. Therefore, solar obser- vation technologies are the key to space weather forecasting studies. Fig.1 shows the schematic relationship between solar surface phenomena and near-Earth space environment disturbances. Solar flares and Coronal Mass Ejections (CME) are thought to be generated when the magnetic-field energy accumulated on the solar surface through magnetic field activities induced by dynamo activity within Fig.1 The Sun and various space environ- the Sun is released by some mechanism. The ment disturbances AKIOKA Maki et al. 3 ation in SEP. Furthermore, high-speed solar bursts with characteristic time-frequency vari- winds may continuously flow out from ation patterns apparent in radio spectral obser- regions called coronal holes without being vations (called Type-Ⅱ and Type-Ⅳ bursts) are confined to the solar atmosphere depending on often found to accompany solar flares accom- the solar-surface magnetic field distribution, panied by eruptions. Thus, these bursts may and these high-speed solar winds are also a serve as an important indicator for occur- major cause of geomagnetic disturbances. rences of geomagnetic storms and proton events. 1.2 Space Weather Forecasting and Information regarding the scale, morphol- Solar Observation ogy, and change of the active region on the Space weather forecasting involves the solar surface is required to assess the hazards monitoring of the Sun and various space envi- of solar activity. It has been empirically deter- ronments and prediction of future conditions. mined that flares have a high probability of Near-Earth space environment disturbances occurring in Emerging Flux Regions (EFR), have their origin in solar-surface phenomena, where new magnetic flux rope rises continu- such as solar flares. In recent years, satellite ously from below the photosphere, and in observations have revealed that CME also regions with complex magnetic field struc- plays an important role[1]. For example, flares tures[3]. Furthermore, major solar flares are displaying a wide region of brightenings in highly likely to occur where regions of oppo- monochromatic imaging for analysis of chro- site polarities meet, especially when accompa- mospheric absorption lines such as Hα-lines nied by a delta configuration, in which the (referred to as "two-ribbon" flares since, in sunspot groups of opposite polarities are in many cases, they appear as two bright lines) close proximity and contained in a single often accompany filament eruptions and penumbra. The solar-surface magnetic field CME, and are mainly categorized as LDE- must be observed with higher precision using type flares of long duration (Fig.2). Com- such instruments as magnetographs and pared to impulsive flares of short duration, the Stokes polarimeters that measure the Zeeman LDE-type flares have been known to be more effect of absorption lines by polarization likely to trigger geomagnetic storms and observations. Also, it is believed that the fib- strong SEP events[2]. Furthermore, radio rils, thread patterns present in the Hα-mono- Fig.2 Solar flare on Apr. 10, 2001 observed Fig.3 Fine structure of sunspot groups seen by high-definition Hαsolar telescope in monochromatic H -line imaging (at Hiraiso) α 4 Journal of the Communications Research Laboratory Vol.49 No.4 2002 chromatic image, reflect the magnetic field 2 High-Definition HαSolar Tele- structure of the chromosphere, and may pro- scope vide valuable insights in determining the mag- netic field structure. The EFR appears as an The high-definition Hαsolar telescope is a arch filament system in an Hα-monochromatic spectral imaging system that uses a birefrin- image, due to the magnetic field lines in the gence interference filter with variable wave- emerging dipole field being observed as lengths (Lyot filters). The system has been absorption materials with a loop-shap. There- under development since 1991, and in July fore, high-resolution Hα-monochromatic 1994, it was incorporated into routine observa- images may be regarded as a powerful tool for tion activities. By acquiring images near an evaluating the complexity of the magnetic absorption line of hydrogen Balmerα, it is field structure on the solar surface in the hori- possible to obtain images of the lower chro- zontal direction. mosphere (at an altitude of approximately With the development of the SOHO (Solar 3,000 km from the solar surface). Further- and Heliospheric Observatory) by NASA and more, the Doppler shift can be measured by the ESA, significant advances have been made alternating the image acquisition between both in observation of the CME, which drives SEP ends of the absorption line and calculating the events and geomagnetic storms. In particular, difference. Thus, this instrument enables us to the observation data of LASCO/C3, which has estimate the velocity of the radial motion of an observation range of 30 times the solar the upper solar atmosphere. The system radius, has not only advanced scientific design and the present state of its operation research on the Sun and the heliosphere, but are briefly given below. has greatly improved the level of space weath- er forecasting overall[4]. The interactions 2.1 System Outline between the plasma and magnetic field on the The main specifications of the high-defini- solar surface are considered as a key factor in tion Hαolar telescope are given in Table 1. the origin of CME and flares generated in the The Carl Zeiss 15-cm refracting telescope is outer part of the solar atmosphere; i.e., in the used as the basic platform for the system, and corona and the chromosphere. For space various improvements and modifications have weather forecasting, as well as solar physics been made to it, such as the addition of auto- studies, it is necessary to cover the entire matic operation functions and improvements active region of the Sun by visible wavelength in performance. For example, to reduce the observations of the solar surface and X-ray, deterioration of the acquired image due to ultraviolet, and radio observations of the upper temperature perturbations and non-uniform solar atmosphere. temperature distribution in the lens cell and This paper will introduce the optical tele- the lens by the heat from the Sun, a heat-rejec- scope (high-definition Hαsolar telescope) and tion filter with an effective diameter of 150 the radio spectrometer (broadband solar radio mm was inserted in front of the objective lens, observation system) that has been developed and the cells of the filter and lens were all by the Hiraiso Solar Observatory of CRL and painted white. The system was also provided is currently employed in routine observation for space environment information service Table 1 Primary specifications of high defini- duties and space weather forecasting studies. tion Hα telescope The details of the development of this system have already been presented in our quarterly journal (Journal of the Communications Research Laboratory), and readers may refer to past reports for technical details[5][6]. AKIOKA Maki et al. 5 with a function for switching the field-of-view The telescope system and the focal plane by shifting the Sun guiding telescope. package are controlled by the telescope control The focal plane package, which consists of system developed on personal computers and the Lyot filter, field lens, full-disk reducing filters. Each actuator and control system are lens, full-disk close-up selection stage, and GP-IB-interfaced and connected to worksta- CCD camera, is set on a base fixed to the tions via optical RS-232 C link. The software polar axis. The Lyot filter is a narrow-band is written in Microsoft Quick-Basic code. filter that utilizes the birefringence of crystals such as calcite, and the transmission width for this system is 0.25Å. By inserting or remov- ing the Polaroid in the final stage, it is possi- ble to select the transmission width (0.25Å/0.5 Å). The transmitted wavelength can be changed by rotating the retardation plate with a motor. The angle of rotation is read by the potentiometer that is coupled with the gear of the retardation plate. The imaging system can be switched between full-disk view (1.1 arc sec./pixel) and close-up view (0.64 arc sec./pixel).
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