This spectrum was accumulated over a three-day period (these values are uncorrected for efficiencies). The MTOF sensor was set in a mode that was optimized for observing solar wind species with masses above that of sulfur. This is why the peaks for Calcium (mass 40) and Iron (mass 56) are so dominant. Oxygen (mass 16) is actually the most abundant heavy element in the solar wind, and the true Iron (mass 56) abundance would be roughly 10% that of Oxygen.
In this color version of the figure, the peaks that are simply line-drawn are the elements commonly observed by in-situ solar wind experiments: Carbon (mass 12 amu), Oxygen (mass 16 amu), Neon (mass 20), Magnesium (mass 24), Silicon (mass 28), and Iron (mass 56).The elements and isotopes for which SOHO MTOF has given the first in situ spacecraft solar wind observations are in red. These include the Silicon isotopes (masses 29, 30), the element Phosphorus (mass 31), a Sulfur isotope (mass 34), the element Chlorine (mass 35) and its isotope (mass 37), an Argon isotope (mass 38), the Calcium isotopes (masses 42 and 44), the element Titanium (mass 48), the element Chromium (mass 52) and its isotope (mass 53), the Iron isotope (mass 54, there is also a shoulder for mass 57), the element Manganese (mass 55), the element Nickel (mass 58) and its isotopes (masses 60, 62).
[For all but Argon 38, these are the first in situ solar wind observations by any means. There was an observation of Argon 38 in the Apollo foil experiments.]
The elements and isotopes that are shaded green are not observed routinely by conventional solar wind experiments. These include the element Nitrogen (mass 14), a Neon isotope (mass 22), the element Sodium (mass 23), the Magnesium isotopes (masses 25, 26), the element Aluminum (mass 27), the Sulfur element (mass 32), the Argon element (mass 36), and the Calcium element (mass 40). Measurements for Sulfur 32 have been made with the Ulysses SWICS experiment, by accumulating over several months of data, and using extensive fitting techniques (it is not really resolved from Iron and Magnesium). Neon 22 and Argon 36 were measured by the Apollo foil experiments. The remainder were first observed in the in situ solar wind by the MTOF prototype sensor, which was launched on WIND about 13 months before SOHO. However, MTOF has a major advantage over its prototype regarding temporal resolution, because of its higher collection power, as discussed in the sensor description.
MTOF is the most powerful solar wind mass spectrometer flown to date. Its mass resolution represents more than an order of magnitude improvement over conventional instrumentation.
MTOF owes its excellent Mass Resolution capability to a specially designed electric field configuration in its time-of-flight region. An ion's time-of-flight through this region is INDEPENDENT of its initial energy or angle. Sub-nanosecond TOF measurements translate into mass resolutions of a fraction of an AMU.
A prototype of the MTOF sensor (known as 'MASS') was flown on the WIND spacecraft (launched about a year before SOHO), and was able to identify a number of elements and isotopes for the first time. The MTOF sensor incorporates a number of enhancements over its prototype, including position-sensing and signal amplitude measurements, and improved background-rejection techniques. MTOF also generates a 2 to 3 order of magnitude improvement in counting statistics, thanks to: (a) the fact that SOHO always looks in the solar direction (unlike the spinning WIND spacecraft); and (b) a novel entrance deflection system with a very wide bandwidth (400% vs. the more typical 5% in other solar wind instruments), requiring only a few voltage steps to cover the entire energy-per-charge distribution of the solar wind.
The Mass and MTOF sensors were designed and fabricated by the University of Maryland; their entrance deflection systems were built by the University of Bern.