### signal:

Ultrasonic signal
* are merely rapid changes in air pressure.
* tend to reflect off hard surfaces, such as concrete or dry wall.

### signal | visual signal:

Brake light
* Some brake light is part of automobiles
  - cars
  - motorbikes
  - taxis
* visual signal

### signal | visual signal:

Flare
* Many flares occur within complexes of magnetic fields called active regions.
* Most flares develop in a matter of minutes and subside over an hour or two.
* appear as brightening in strong magnetic field regions.
* are a pyrotechnic device and are dangerous and deadly in the wrong, or very young hands
  - bright filaments of hot gas emerging from sunspot regions
  - brighter than the whole Sun in X-rays and in ultraviolet light
  - even more violent rupturing the surface of the sun in just minutes
  - explosive eruptions of energy on the surface of the sun
  - giant releases of energy that can evolve in a few minutes or over several hours
  - in fact difficult to see against the bright emission from the photosphere
  - powerful explosions on the surfaces of many stars
  - short lived explosive brightening of plages
* are very active sunspot sites
  - violent and rapid explosions on the sun
  - violent explosions of hot material occurring in regions around sunspots
* burn at uneven rates and therefore fluctuate in brightness.
* can occasionally cause wildfires
  - therefore provide a significant amount of energy to heat the coronae of active stars
* cause chicken fat to melt.
* emit high-energy atomic particles and all forms of electromagnetic radiation into space.
* happen on flare stars in a similar way to solar flares.
* heat the solar gas to tens of millions of degrees.
* is attacks
  - automotive products
  - devices
  - effusion
  - flies
  - reflection
  - shapes
* occur above sunspots when the energy contained in the magnetic field is suddenly released
  - during periods of solar activity, which typically last a few days
  - much more commonly during the maximum of the solar sunspot cycle
  - near sunspots groups and are caused by tangled magnetic field
* often disturb the atmosphere electrically, thus interfering with radio transmissions.
* produce a high and rapid increase of the density and temperature.
+ Flare star: Types of stars
  * Flares happen on flare stars in a similar way to solar flares. They are magnetic disturbances in the atmosphere of stars. The brightness increases across the spectrum, from X rays to radio waves.

Skyrocket
* are fireworks.
* contain a wooden stick for guidance and stability and rise into the air upon ignition.

Warning light
* help stop accidents.
* visual signal
+ Visibility: Weather
  * Visibility less than  is often called zero. When it is this low, roads may be closed, or lights and signs are turned on to warn drivers. These are put in areas that often have very low visibility. Warning lights help stop accidents. Automobile crashes with many cars happen often in places like this if the warning lights and signs are not put up.

Signature
* are icons
  - important on documents for many reasons
  - names
  - part of books
  - sheets
  - short text files which automatically attach to the end of any email message sent
  - the text that people place at the end of messages
  - tunes
* way of linking cases.

### signature:

Autograph
* also occur on books, baseballs, and other items.
* are a large collectibles market and a big part of the music memorabilia market
  - very significant part of the sports memorabilia market
  - our connections to the stars
  - signatures
  - writing
* come in different forms with different values.

Digital signature
* are implemented using cryptography.
* is an alternative for a handwritten signature
  - authentication of an electronic record by a person using electronic means
  - one of the great innovations in the history of cryptography