In what part of electromagnetic spectrum does the sun radiate maximum energy How does this compare to Earth?

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All matter in the universe that has a temperature above absolute zero (the temperature at which all atomic or molecular motion stops) radiates energy across a range of wavelengths in the electromagnetic spectrum. The hotter something is, the shorter its peak wavelength of radiated energy is. The hottest objects in the universe radiate mostly gamma rays and x-rays. Cooler objects emit mostly longer-wavelength radiation, including visible light, thermal infrared, radio, and microwaves.

The Sun’s surface temperature is 5,500° C, and its peak radiation is in visible wavelengths of light. Earth’s effective temperature—the temperature it appears when viewed from space—is -20° C, and it radiates energy that peaks in thermal infrared wavelengths. (Illustration adapted from Robert Rohde.)

Incandescent light bulbs radiate 40 to 100 watts. The Sun delivers 1,360 watts per square meter. An astronaut facing the Sun has a surface area of about 0.85 square meters, so he or she receives energy equivalent to 19 60-watt light bulbs. (Photograph ©2005 Paul Watson.)

The surface of the Sun has a temperature of about 5,800 Kelvin (about 5,500 degrees Celsius, or about 10,000 degrees Fahrenheit). At that temperature, most of the energy the Sun radiates is visible and near-infrared light. At Earth’s average distance from the Sun (about 150 million kilometers), the average intensity of solar energy reaching the top of the atmosphere directly facing the Sun is about 1,360 watts per square meter, according to measurements made by the most recent NASA satellite missions. This amount of power is known as the total solar irradiance. (Before scientists discovered that it varies by a small amount during the sunspot cycle, total solar irradiance was sometimes called “the solar constant.”)

A watt is measurement of power, or the amount of energy that something generates or uses over time. How much power is 1,360 watts? An incandescent light bulb uses anywhere from 40 to 100 watts. A microwave uses about 1000 watts. If for just one hour, you could capture and re-use all the solar energy arriving over a single square meter at the top of the atmosphere directly facing the Sun—an area no wider than an adult’s outstretched arm span—you would have enough to run a refrigerator all day.

The total solar irradiance is the maximum possible power that the Sun can deliver to a planet at Earth’s average distance from the Sun; basic geometry limits the actual solar energy intercepted by Earth. Only half the Earth is ever lit by the Sun at one time, which halves the total solar irradiance.

Energy from sunlight is not spread evenly over Earth. One hemisphere is always dark, receiving no solar radiation at all. On the daylight side, only the point directly under the Sun receives full-intensity solar radiation. From the equator to the poles, the Sun’ rays meet Earth at smaller and smaller angles, and the light gets spread over larger and larger surface areas (red lines). (NASA illustration by Robert Simmon.)

In addition, the total solar irradiance is the maximum power the Sun can deliver to a surface that is perpendicular to the path of incoming light. Because the Earth is a sphere, only areas near the equator at midday come close to being perpendicular to the path of incoming light. Everywhere else, the light comes in at an angle. The progressive decrease in the angle of solar illumination with increasing latitude reduces the average solar irradiance by an additional one-half.

The solar radiation received at Earth’s surface varies by time and latitude. This graph illustrates the relationship between latitude, time, and solar energy during the equinoxes. The illustrations show how the time of day (A-E) affects the angle of incoming sunlight (revealed by the length of the shadow) and the light’s intensity. On the equinoxes, the Sun rises at 6:00 a.m. everywhere. The strength of sunlight increases from sunrise until noon, when the Sun is directly overhead along the equator (casting no shadow). After noon, the strength of sunlight decreases until the Sun sets at 6:00 p.m. The tropics (from 0 to 23.5° latitude) receive about 90% of the energy compared to the equator, the mid-latitudes (45°) roughly 70%, and the Arctic and Antarctic Circles about 40%. (NASA illustration by Robert Simmon.)

Averaged over the entire planet, the amount of sunlight arriving at the top of Earth’s atmosphere is only one-fourth of the total solar irradiance, or approximately 340 watts per square meter.

When the flow of incoming solar energy is balanced by an equal flow of heat to space, Earth is in radiative equilibrium, and global temperature is relatively stable. Anything that increases or decreases the amount of incoming or outgoing energy disturbs Earth’s radiative equilibrium; global temperatures must rise or fall in response.

When you hear the word "light," what comes to mind? For most people, the answer is sunlight. The sun constantly emits light.

Light continuously reaches the earth from the sun. You might also say, "The sun sends electromagnetic energy as light." The electromagnetic waves emitted by the sun are of a broad spectrum ranging from X-rays with a wavelength of 2 nanometers to radio waves with a wavelength of 10 meters. The most intense of these to reach the earth's surface is visible light, with a wavelength around 500 nanometers.

The energy the earth receives from the sun is called the "solar constant," which is defined as 2 calories per square centimeter per minute. Based on meters squared, the solar constant is equivalent to 1.4 kilowatts, or one electric heater. The energy reaching the earth's surface is less than this value, owing to such factors as atmospheric absorption, but the sun is nevertheless a major source of energy for the earth.

The blue curve shows the solar radiation at the top of the atmosphere. The yellow curve shows the radiation at the earth's surface, and is lower due to reflection, absorption, and scattering in the atmosphere on the way down. Several absorption bands, where the atmosphere absorbs preferentially, appear as downward spikes in the yellow curve. Each absorption band corresponds to a specific gas in the atmosphere.

The Stefan-Boltzmann law governs the radiation of energy by a black body: M = {emissivity} * T4. Emissivity is a constant (perhaps a function of wavelength), so the energy radiated increases as the 4th power of temperature. The sun, T=6000°K, emits vastly more energy than the earth at T=300°K.

The wavelength of maximum energy radiated by a body is given by Wien's displacement law: Lambda = A / T.   A is a constant, so as temperature increase the wavelength decreases. The sun, T=6000°K, emits primarily in the visible range, while the earth, T=300°K, emits primarily in the thermal IR range.

The atmosphere greatly affects remote sensing, dictating what portions of the electromagnetic spectrum we use.

A body emits radiation as a result of its temperature. A surface reflects energy according to its albedo; with a high albedo, most of the energy is bounced off the surface without change. Low albedo surfaces absorb the energy, which will warm the object and eventually be reradiated at the longer wavelength characteristic of the body's temperature.  Albedo, like emissivity, is a function of wavelength.  The sum emits with a peak in the visible, which the earth absorbs and then reradiates with a peak in the thermal IR portion of the spectrum. This "greenhouse effect" heats the atmosphere from below, as the visible light from the sun passes through the atmosphere with little impact other than scattering, mostly in the blue wavelengths,  while the atmosphere absorbs the thermal energy reradiated by the earth. Reflected energy requires an outside source, usually the sun, and thus works only during the day.

Remote sensing generally uses wavelengths in micometers (µm), and considers UV, visible, NIR, SWIR, TIR, and microwave.  UV, VIS, and NIR are emitted by the sun and reflected to various degrees from the earth, based on the albedo at a particular frequency.  The TIR and microwave are emitted by the earth.  All of the energy wavelengths can be affected by the atmosphere, and limit what the satllite can detect.

Black body (emissivity = 1) radiation at various temperatures.  These curves reflect the physics in Kirchhoff's law, Wien's displacement law, and The Stefan–Boltzmann law.The sun, with a temperature of about 6000K, emits a lot of energy, with the peak in the visible portion of the spectrum.  This energy, after passing through the atmosphere, reflects from the surface and can be measured by satellite sensors.The earth, with a temperature of about 300K, emits much less energy, and its peak emission is diplaced to much long wavelengths in the thermal IR part of the spectrum, offscale in this diagram.  Becasue of the small energy, the TIR bands on Landsat have much lower spatial resolution.  At one time they came in 120 m pixels sizes, which made remote sensing software deal with the differences.  As storage costs came down, the TIR bands were distributed with with same 30 m pixel size as the other bands, but they are noticeably more blurry than the other bands.  Weather satellites, which seek very accurate temperature readings, have much larger pixel sizes, on the order of a kilometer, which is acceptable because of the small scale at which atomosphere and ocean operate. Landsat 7 and 8 have a panchromatic band, with a much wider range of wavelengths collected.  This collects much more energy, because there is a much  larger area under the curve, and allows Landsat to have 15 m pixels for this band. Figure from wikipedia.

Last revision 11/2/2021