PDS_VERSION_ID = PDS3 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ICE INSTRUMENT_ID = ULECA OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ULTRA LOW ENERGY CHARGE ANALYZER" INSTRUMENT_TYPE = "ELECTROSTATIC ANALYZER" INSTRUMENT_DESC = " Instrument Overview =================== The ISEE-1 and ISEE-C instruments have been designed to measure the elemental abundances, charge state composition, energy spectra, and angular distributions of energetic ions in the energy range 2 keV/charge to 80 MeV/nucleon and of electrons between 75 and 1300 keV. By covering the energy range between solar wind and low-energy cosmic rays the instrument will fill a gap in the knowledge especially of the nuclear and ionic composition of solar, interplanetary, and magneto- spheric accelerated and trapped particles. The instrument consists of three different sensor systems: ULECA is an electrostatic deflection analyzer system with rectangular solid-state detectors as energy deter- mining devices, its energy range is -3 to 560 keV/charge; the ULEWAT is a double dE/dX versus E thin-window flow-through proportional counter/solid-state detector telescope covering the energy range from 0.2 to 80 MeV/nucleon (Fe); the ULEZEQ sensor consists of a com- bination of an electrostatic deflection analyzer and a thin-window dE/dX versus E system with a thin-window proportional counter and a position- sensitive sofid-state detector. The energy range is 0.4 MeV/nucleon to 6 MeV/nucleon. While the ULECA and the ULEWAT sensors are de- signed mainly for interplanetary and outer magnetospheric studies, the ULEZEQ sensor will also obtain composition data in the trapped radia- tion zone. 65 rates and pulse-height data can be obtained with sector- ing in up to 16 sectors. SCIENTIFIC OBJECTIVES ====================== The low-energy portion of interplanetary and magneto- spheric particles (energies 1 keV/nucleon to several hundred keV/nucleon) has been virtually unexplored to date. Measure. ments in this region, however, are expected to provide sensitive probes of solar, interplanetary, and magnetospheric phenomena. A. SolarandlnterplanetaryPhenomena 1) Solar Flare Acceleration Mechanism: The mechanism by which ions are accelerated in flares is not presently known. Important information on this mechanism, however, is avafl. able from studies of the energy spectra of the particles which the mechanism produces. This is particularly true in the case of heavier ions at low energies. These particles are not ex- pected to be fully stripped of their electrons. Thus in the same energy range different ions cover different rigidity in ter-vals. By examining, then, the energy spectra of many ions at low energies, and also electrons, we will place stringent constraints on the rigidity dependence of the flare accelera- tion process. 2) The Location of the Flare Acceleration Region: Low- energy ions also provide in their charge states infon-nation on the region in the corona where they are accelerated. The charge of solar particles is frozen-in as the particles leave the corona; the plasma density in interplanetary space is too low to cause additional ionization or recombination. Thus the charge composition, particularly of very low energy ions, can reflect the temperature of coronal electrons in the region where the particles are accelerated, propagate, or are stored, For example, if the ions are accelerated in the flare site itself, we should expect a high degree of ionization, appropriate to the high temperature in the flare. In contrast, if the particles are accelerated in the surrounding corona by, e.g., plasma dis- turbances emitted from the flare, we should expect a charge composition more similar to that of the solar wind. 3) Coronal Propagation and Storage: Low-energy ions also provide some of the most detailed information on coronal propagation mechanisms, and coronal storage. Measurements in the broad rigidity range covered by partially stripped ions at low energies, and also by low-energy electrons, will pro- vide stringent tests of the current idea that coronal propaga- tion is rigidity-independent. Further, ionization loss which is a consequence of extended coronal storage is most evident at low energies, where it should produce in the differential energy spectra a systematic flattening which depends on the charge squared to mass ratio. From the flattening observed in the spectra of different ions, it should be possible to deter- mine the time during which the particles are stored and/or the coronal density and thus the location of the region where the storage occurs. 4) Compositional Variations in Solar Flares: The composi- tion of energetic flare particles is known to vary widely from flare to flare, particularly at low energies. The cause of this variability is not presently known. It may result from the fact that heavier ions, which may not be fully stripped of their electrons, can exhibit different rigidities in different flares, and thus will respond differently to the flare acceleration process. It may be also that the variations reflect composi- tional anomalies in the coronal material from which the particles are accelerated. Our investigation, which can deter- mine the charge states of low-energy ions, will probe the former explanation in detail. From these charge-state mea- surements, which can indicate the coronal conditions where the particles are accelerated, and from our general ability to observe flares in considerable detail, we will also provide in- formation on the latter possibility. Of particular interest in the study of compositional varia- tions in solar events are the so-called 3He-rich event, in which the 3He/4He ratio can exceed unity. These flares are one of the more dramatic examples of compositional anomalies since the coronal abundance is 3He/4He < 10**-3 . These flares also have the peculiarity that there is no accompanying increase in deuterium and tritium, which are equal byproducts of 3He in spallation reactions, and there is a general enhancement in heavier elements, particularly in iron. Our investigation will extend the measurements of 3 He and heavier ions in these flares to much lower energies than has been possible to date. 5) Interplanetary Propagation: The current theory for en- ergetic particle interactions with the magnetic field in the solar wind is inadequate in that it predicts more pitch-angle scatter- ing than is observed. These differences are most pronounced, and thus most easily studied at low energies (< 1 MeV/nucleon). We will make a detailed study of propagation at low energies by observing the anisotropies of protons and helium through- out our energy range, as well as the time profiles of different ion species and electrons. The former measurement is a sensitive indicator of the extent of the scattering; both mea- surements provide information on its rigidity dependence. 6) Interplanetary Acceleration: It appears from studies in recent years that the majority of co-rotating particle streams are the result of interplanetary acceleration, as opposed to of direct solar origin. The origin of the particles, however, is not presently known. It may be that the particles are accelerated out of the solar wind. It is possible also that the particles enter the solar wind as energetic solar particles and receive here an additional acceleration. Our measurements of the spectra, anisotropies, and charge states of ions, over an es- sentially continuous energy range from the solar wind to high- energy particles, should provide stringent tests of these pos- sible origins. If these particles are accelerated out of the solar wind, their charge states should be those of solar wind ions, and their spectra a continuous extension of the solar wind distribution. Particles which originate as more energetic solar particles, in contrast, may be more highly ionized and may exhibit a behavior at higher energies which is uncorrelated with that closer to solar wind energies. 7) The Anomalous Cosmic-Ray Component: From 1972 on, a component with the anomalous composition of only helium, nitrogen, oxygen, and neon has been observed in the cosmic-ray flux at energies 1-30 MeV/nucleon. The origin of this component is presently being debated, although there is mounting evidence that it results from interstellar neutral particles which are swept into the heliosphere and ionized and accelerated in the solar wind. This component appears to be a feature of solar minimum conditions. Our observations over the next few years will record the behavior of this component as solar activity increases with the onset of the new solar cycle. 8) Correlated Studies with Deep-Space Missions: Our in- vestigation will also provide measurements at earth for cor- related studies of galactic cosmic-ray modulation and solar flare propagation with deep-space missions such as Pioneer and Voyager. Detectors ========= The ULECA (Utra Low Energy Charge Analyzer) sensor incorporates techniques of electrostatic deflection and a total energy measurement to provide the charge composi- tion of ions in the energy range 2 to 560 keV/charge. ULECA is a design of the University of Maryland ECA (Energy Charge Analyzer) instrument flown on IMP 7 and 8 satellites . Low-energy ions pass through a multisht focusing collimator and enter one of three deflection regions designed by L, M, and MP. Seven rectangular surface-barrier (Au-Si) solid-state detectors are placed at fixed positions at the exit of the de- flection regions, each defining a given energy per charge window. The output of each of these detectors is pulse- height analyzed provided the solid-state anticoincidence detectors are not triggered. A residual background (due to cosmic-ray produced secondaries) and the geometrical factor of the collimator determine the minimum flux which can be measured by each detector (see table below). The majority of penetrating particles is eliminated from analysis by the solid-state anticoincidence detectors. TABLE RATE CHANNEL CHARACTERISTICS ULECA SENSOR Rate Readout Designation Particle Energy Range Period Conversion Factor Minimum Flux (N/S) 1 Type 2 (keV/charge) (S) 3 CM**2-sr-keV CM**2-sr-s-keV ------------ -------------- charge**-l charge**-l Ll (S) > 3 2.88-3.084 16 581 290 8.68-9.29 191 95 L2 (S) Q > 1 8.14-9.384 16 94 47 26.4-30.5 28 14 Mll (N) Q = 1 27.4-32.7 8 18.5 1520 (S) 32 5 M12 (N) Q = 2 27.4-32.7 8 18.5 8 (S) 32 3 M13 (N) Q > 2 27.4-32.7 32 18.5 61 (S) 64 15 M21 (N) Q = 1 53-69 8 5.35 17 (S) 64 1.3 M22 (N) Q = 2 53-69 8 5.35 6 32 0.7 M23 (N) Q > 2 53-69 64 5.35 2.2 M31 (N) = 1 103-140 8 2.52 298 32 0.7 M32 (N) Q = 2 103-140 8 2.52 0.3 M33 (N) Q > 2 103-140 32 2.52 0.2 MP12(N) 1,2 105-560 64 0.33 3.5 MP3L 3 105-560 64 0.33 0.2 MPI L = 1 105-560 32 0.33 3.3 MP2L Q = 2 105-560 32 0.33 0.1 1 N = nonsectored; S sectored into 8-45 deg sectors in ecliptic plane. 2 Q = charge state of ion. 3 At high bit rate divide period by 4. 4 L1, L2 energy range assumes medium voltage mode. The major effect not included is the secondary electron background in the solid-state detector. This effect is caused by the interaction of high-energy particles in the spacecraft. Our experience with similar detector systems on IMP's 7 and 8 indicate that this will not be an important effect for mag- netospheric events. In the L deflection region, a variable power supply steps the deflection voltage in 32 logarithmic increments from 600 to 1550 V, stepping once every 5 spin periods (16 s). The voltage range may be changed up or down by 50 percent via ground con-unand. Two low-noise (1 5-keV energy threshold) rectangular solid-state detectors Ll and L2 measure the energy and record the counting rate of deflected ions for each voltage step. The relative energy per charge windows delta E/E are 0.07 and 0.15 (FWHM), and the energy ranges over which measurements are made are 1.8 to 9.5 and 5.1 to 30.5 keV/charge for L1 and L2, respectively. In the M and MP deflection region, two high-voltage supplies are used to provide deflection fields of 1.5 and 6.7 kV/cm, respectively. At the exit of the M section 3 rectangular solid- state detectors, Ml, M2, and M3 measure the fluxes and anisotropies of protons, alpha's and Q > 4 ions in the energy range 25 to 140 keV/charge. These detectors are also pulse- height-analyzed for more detailed charge spectra. In the MP section, a single position-sensitive rectangular Si detector is used to determine the charge spectrum of ions (H to Fe) from 105 to 560 keV/charge. Measured Parameters =================== Each detector is monitored by one or more threshold dis- criminators whose outputs are passed through a system of coincidence-anticoincidence logic which generates 65 dif- ferent counting rates corresponding to a variety of detector combinations, particle energy windows, etc. The number of accumulated counts for any particular rate is stored in one 24-bit register in a 256 register memory. (There are two such memories, identical in all respects, for redundancy. For the most part only one memory is used with the second remaining as a backup, with data storage able to be switched between the memories by command.) Individual rates are read out at predetermined intervals and logarithmically com- pressed to either 10 or 12 bits and inserted into the telemetry stream. Some rates are accumulated continuously over all directions during the spacecraft spin (omnidirectional rates), others are sorted into 8 directional angular sectors so as to measure flux anisotropies (sectored rates). Of these rates, the BASIC rates are of special importance. Their logic require- ments are identical to the ones required to trigger individual pulse-height analyzed (PHA) events. Absolute fluxes can therefore, be computed from PHA events. Due to the large number of rate channels generated, the telemetry of in- dividual rates involves a rather complicated submultiplexing scheme, which cannot be described here. " END_OBJECT = INSTRUMENT_INFORMATION /* /* The INSTRUMENT_REFERENCE_INFO object provides a pointer to /* related reference publications or private communications. Only /* the key is provided in this file. The catalog object which /* provides the full citation is delivered separately. /* /* The INSTRUMENT_REFERENCE_INFO object is repeated once for /* each reference. /* OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "HOVESTADTETAL1978" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END